Fico, who survived an assassination attempt last May, quoted from what he said was a classified report ahead of an opposition-sponsored motion of no-confidence in his government on Tuesday.

His claim coincided with a visit by Hungary’s prime minister, Viktor Orban.

Both leaders have strongly criticised Ukraine’s president and have advocated peace talks with Russia’s Vladimir Putin.

The opposition accused Fico of abusing the SIS to torpedo parliamentary procedure and prevent public scrutiny of his government, which holds a razor-thin majority.

Fico has warned of early elections if his coalition allies do not resolve their differences.

Tuesday’s no-confidence vote in the Slovak parliament was abandoned after the speaker agreed to Fico’s request to hold the entire session in secret.

The opposition promptly walked out.

Fico had insisted he would have to refer to the report – by Slovakia’s SIS civilian intelligence service – during the debate preceding the vote.

“The report was just a bunch of platitudes compiled from emails,” said Tomas Valasek, deputy leader of the liberal opposition Progressive Slovakia, which helped to file the no-confidence motion.

“But the act of classifying it was important, because it allowed them to classify the entire debate on the government’s failings,” he told the BBC from Bratislava.

“It was basically a procedural trick to muzzle the opposition. That’s all there is to the report,” he went on.

Speaking to reporters shortly after the debate, Fico accused the opposition of trying to overthrow his democratically elected government, a coalition of leftist, nationalist and populist parties.

“I can’t give you names and I can’t discuss the circumstances, but I can tell you in all seriousness that the Slovak opposition is planning a Maidan,” he said.

“The Slovak opposition is making plans to occupy government buildings. It intends to stop the authorities from carrying out their official duties,” the prime minister added.

The SIS later released a statement saying it had obtained serious information about a “long-term, organised influence operation with the aim of destabilising the Slovak Republic”.

The emails which allegedly featured in the classified report were later leaked to the daily SME newspaper.

Evidence of a secret coup plot looks thin.

The emails were from an NGO in the city of Banska Bystrica called “Not In Our Town”, which has been active since 2013. The emails do propose an escalating series of non-violent public protests and blockades of public buildings, with three stated goals:

• To force the government to act in accordance with the law and democratic principles
• To reject any form of collaboration with Russia and support Ukraine
• To maintain Slovakia’s pro-European and pro-Atlantic orientation.

But Not In Our Town said there was nothing secretive about the plans – they were written with the help of ChatGPT and were emailed to some 140 organisers.

“It is stressful, but on the other hand it’s rather ridiculous,” said one of the group’s coordinators called Martina, who preferred not to give her full name.

“We’ve put the emails on our Facebook page because there’s nothing secret about them. It’s crazy,” she told the BBC.

She had not, she said, been approached by the intelligence services.

The opposition has little faith in the impartiality of the SIS. One of Robert Fico’s first acts when he returned to office in 2023 was to appoint Pavol Gaspar, son of former police chief Tibor Gaspar, to run the agency.

Tibor Gaspar was forced to resign as chief of police following the murder of investigative journalist Jan Kuciak in 2018, a seismic event in Slovak politics that also forced Mr Fico to abandon his third term as prime minister.

Gaspar is now an MP in Fico’s leftist Smer party, and has just returned from a controversial visit to Moscow. His son says it is wrong to cast aspersions on the professionalism of the SIS.

Meanwhile, Fico will oversee an emergency session of the security council called for Thursday morning to discuss the findings of the “secret report”.

He promised “significant preventative measures” to prevent what he said was an alleged coup against his government.

He claimed that a group of US-funded experts involved in anti-Yanukovych protests in Ukraine and more recent demonstrations in Georgia, was already operating in Slovakia. It was being closely monitored by the Slovak intelligence services, he asserted.

Tens of thousands of Slovaks have demonstrated in recent months against the government’s alleged curtailing of the rule of law and media freedom, the renewal of relations with Russia and a halt in support for Ukraine.

The latest round of nationwide protests will be held on Friday.

On 1 January, Kyiv shut off a pipeline that for decades was used to supply Central Europe with Russian natural gas.

Slovakia had been the main entry point and the country now stands to lose millions of euros in transit fees.

The UN’s refugee agency (UNHCR) estimated last month that there were 130,530 Ukrainian refugees in Slovakia out of 6,813,900 globally.

Fico – who in December made a surprise visit to Moscow for talks with Russia’s President Vladimir Putin – described Kyiv’s move as “sabotage”.

The prime minister of the EU state said he would propose halting electricity exports to Ukraine and also “sharply reducing” financial support for Ukrainians who have found shelter in Slovakia.

He said there was no risk of Slovakia itself suffering from gas shortages, as it had already made alternative arrangements.

But Fico added that Ukrainian President Volodymyr Zelensky’s decision to turn off the taps would deprive Slovakia of 500m euros (£415m; $518m) in transit fees from other countries.

He said his party was ready to debate “halting supplies of electricity” and the “significant lowering of support for Ukrainian citizens in Slovakia”.

“The only alternative for a sovereign Slovakia is renewal of transit or demanding compensation mechanisms that will replace the loss in public finances,” he added.

Last month Zelensky accused Fico of helping Putin to “fund the war and weaken Ukraine”.

“Fico is dragging Slovakia into Russia’s attempts to cause more suffering for Ukrainians,” the Ukrainian president had said.

Poland has offered to support Kyiv in case Slovakia cuts off its electricity exports – supplies that are crucial to Ukraine, whose power plants come under regular attack from Russia.

Poland’s government called Ukraine’s shutdown of Russian gas supplies “another victory” against Moscow while the European Commission said the EU had prepared for the change and most states could cope.

Moldova, which is not in the EU, is already suffering shortages.

Russia can still send gas to Hungary, Turkey and Serbia through the TurkStream pipeline across the Black Sea.

Ukraine pumps Russian natural gas through its territory to several European countries, including Slovakia, Moldova and Hungary.

But it is expected to halt the flow when an existing transit deal, which was signed before Russia invaded Ukraine in 2022, expires at the end of the year.

Fico, who visited Russian President Vladimir Putin in Moscow earlier this week, said on Friday that Slovakia would consider reciprocal measures against Ukraine – such as halting back-up electricity supplies – if Kyiv cuts off the transfers as of January 1.

“It appears that Putin gave Fico the order to open the second energy front against Ukraine at the expense of the Slovak people’s interests,” Zelenskyy wrote in a social media post on Saturday.

“Fico threats to cut off Ukraine’s emergency power supply this winter while Russia attacks our power plants and energy grid can only be explained by this,” he said, adding that Fico was “dragging Slovakia into Russia’s attempts to cause more suffering for Ukrainians”.

Slovakia wants to maintain Russian gas supplies via Ukraine, saying alternative routes would hike costs and harm its transit operations while causing the country to lose 500 million euros ($521m) in fees.

Kyiv has said it will not sign any new deal for gas transit with Moscow as the war between the two countries drags on.

The Ukrainian government has been forced to import electricity from several of its neighbours since Russia began hitting its power grid in late 2022, damaging or destroying much of the country’s non-nuclear generation capacity.

Russian gas accounted for less than 10 percent of the European Union’s gas imports in 2023. In 2021, a year before the Ukraine invasion began, it made up more than 40 percent.

However, EU members in Eastern Europe still depend largely on Russian gas for geographical and political reasons.

Zelenskyy said Slovakia currently accounted for 19 percent of Ukraine’s power imports, and that Kyiv was working with its EU neighbours to bolster the supply.

“Slovakia is part of the single European energy market and Fico must respect common European rules,” the Ukrainian president said on Saturday.

Meanwhile, Slovakia’s prime minister said on Friday his country could serve as a meeting place for possible talks on ending the war in Ukraine.

Fico is one of only a handful of European leaders who have remained close to the Kremlin, and his meeting with Putin in Moscow on December 22 provoked an angry reaction from Ukraine.

“If anyone turns to us and is interested in organising peace talks in Slovakia about the conflict between Ukraine and Russia, they can count on our hospitality,” Fico said in a video message published on Facebook.

Fico said he had discussed the proposal with Putin during his surprise visit to the Russian capital and was pleased with the positive reaction.

Fico – a vocal critic of the European Union’s support for Kyiv in the war – said they discussed supplies of Russian gas to Slovakia – which his country relies on.

A deal with Russian gas giant Gazprom to transit energy through Ukraine to Slovakia is due to expire at the end of this year.

“Top EU officials were informed about my journey and its purpose… on Friday,” Fico wrote on Facebook.

Fico said the meeting in Moscow was a reaction to Ukrainian President Volodymyr Zelensky telling EU leaders that Ukraine remains opposed to Russian gas being piped through its territory.

The Slovakian PM, who survived being shot earlier this year, also said he had a “long conversation” with Putin and the two “exchanged views on the military situation in Ukraine”.

Both discussed “the possibilities of an early, peaceful end of the war” and mutual relations between Russia and Slovakia, Fico wrote on Facebook.

Slovakia and Hungary, which both depend on Russian gas, have raised concerns about the prospect of supplies being interrupted.

In October 2023, when Fico became prime minister again, he ended Slovakia’s military aid to Ukraine.

But, he has insisted he wants to be a “good, friendly neighbour” to Ukraine.

Fico’s meeting with Putin came as the leaders of Italy, Sweden, Greece and Finland met on Sunday for a security summit.

Speaking afterwards, Finland’s Prime Minister Petteri Orpo said Russia was a “permanent and dangerous threat” to the EU.

He also stressed the need for increased defence spending and support for Ukraine.