Hard-right populist George Simion will contest Romania's presidential election result.
The 38-year-old had been the favourite to win at the polls - but unexpectedly lost to pro-Western candidate Nicusor Dan over the weekend.
Mr Simion now claims to have hard evidence of foreign interference in the vote.
He said in a statement: "We now have irrefutable evidence of meddling by France, Moldova, and other actors, in an orchestrated effort to manipulate institutions, direct media narratives, and ultimately impose a result that does not reflect the sovereign will of the Romanian people."
In the first round, the Trump supporter took 40.96% of the votes, putting him almost 20 points ahead. But the second round saw Mr Dan, the mayor of Bucharest, end up with 54% of the votes, while Mr Simion secured 46%.
A voter turnout of 64.7% in Sunday's ballot, more than in any Romanian election of the past 25 years, is thought to have benefited Mr Dan.
Mr Simion has submitted a formal request to Romania's Constitutional Court, calling for it to annul the result.
Mr Dan, a 55-year-old mathematician, ran as an independent candidate and his pledges included a clampdown on rampant corruption and maintaining support for Ukraine.
"We need to build Romania together irrespective of who you voted for," he said when his victory was assured.
After exit polls indicated the pro-EU candidate had won, Mr Simion posted on Facebook: "I won!!! I am the new President of Romania and I am giving back the power to the Romanians!" However, he conceded the election later in the evening.
Mr Simion - a nationalist banned from Ukraine, who has advocated for uniting Romania with Moldova - surged to prominence after the country's first attempt to hold its presidential election last year.
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Far-right outsider Calin Georgescu topped the first-round polls, but Romania's highest court disqualified him and annulled the result after allegations of electoral violations and Russian interference.
Mr Simion capitalised on the furore, offering his support to Mr Georgescu and promising to appoint him as prime minister if he secured the presidency.
(c) Sky News 2025: Hard-right populist George Simion to contest Romania's election result
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