France's opposition was plunged into an unprecedented crisis on Monday as the two men fighting to lead the party continued to dispute the results of a botched election.

A second investigation confirmed Jean-François Copé as the new leader of the Union pour un Mouvement Populaire (UMP), the centre-right party of former president Nicolas Sarkozy.

But his rival, former prime minister François Fillon, immediately rejected the result saying the new count was "illegal" and the UMP appeals committee that produced it was biased in favour of Copé.

Copé, 48, was initially declared winner by a margin of just 98 votes after the ballot of party members eight days ago. Shortly afterwards Fillon, 58, claimed votes from three overseas districts had been "forgotten" and would have given him victory.

For the past week, the two sides have ripped the UMP apart, accusing each other of cheating and election fraud while party heavyweights called for calm and tried to referee the dispute. One French newspaper described it as "live suicide".

As a final attempt to reconcile the rival camps, party grandee Alain Juppé was brought in to mediate. He threw in the towel on Sunday evening after his peace efforts failed.

It was reported yesterday that at a peacemaking lunch yesterday with Fillon, Sarkozy said holding a new vote would "avoid an escalation of the conflict", a party source told AFP – an account confirmed by both Fillon and Copé loyalists in the party.

The latest result, which appeared to show the hardliner Copé had won by 952 votes out of around 173,000 cast, followed a day of new lows that saw a bailiff sent by a judge to seize contested ballot papers turned away from the UMP headquarters, allegedly by Copé supporters.

The legal move followed a breakdown in talks between Fillon and Copé. The bailiff, sent on the orders of a judge at the Tribunal de Grand Instance in Paris, arrived at the UMP offices in the 15th arrondissement of Paris on Monday morning, to remove voting papers and documentation relating to the ballot after Fillon supporters suggested there was a risk of "manipulation and alteration".

However, shortly after midday, he left empty handed after party officials loyal to Copé were reported to have refused access to the documents.

"The [UMP] leaders have deliberately refused to execute a judicial order ... in politics, contempt for the justice system is a pretty bad augury for the quality of leaders," said Fillon's lawyer François Sureau.

The Fillon camp said it had been forced to act after Copé refused to give it access to the documents. The move was aimed at "conserving" the documents, it said.

"The electoral documents cannot be considered to be safe from manipulations or alterations," said one of Fillon's supporters.

Jean-Pierre Versini-Campinchi, lawyer for the UMP, was due in court on Monday evening to try to delay the seizure of the election documents by the court until the party's appeals commission has finished examining the ballot papers.

Speaking earlier in the day, Juppé said: "It's clear that he is the only one who today has sufficient authority to finally propose a solution, that, as far as I'm concerned, is difficult to see. Can Nicolas Sarkozy calm things down and persuade the two parties back around the table to discuss a solution? That's for him to decide."

Former UMP cabinet minister Nathalie Kosciusko-Morizet launched a petition calling for a re-vote among the 300,000 party members, saying the "political legitimacy" of the first election on 18 November was compromised. "It is too doubtful, too contested," she told Europe 1 radio.

"Nicolas Sarkozy, so far, so near ..." wrote centre-right newspaper Le Figaro.

"Far, very far from the Barnum show that is the UMP, the international conference lecturer Nicolas Sarkozy flew off to Shanghai on Friday where he held a conference on the relations between the European Union and China.

"Since his return on Monday morning, Nicolas Sarkozy was plunged once more in the nightmare of an UMP in shreds. What is the former president of the Republic to do?"

Analysts say Sarkozy, who until now has been careful to keep out of the fray, is primarily concerned over how the outcome of the UMP election will affect his chances of returning to politics if he wishes.

At least one UMP member of parliament has threatened to leave the party if Copé refuses to stand down. Michel Piron said he would not "stay in a UMP presided over by a president who is contested and contestable".

Party treasurer Dominique Dord announced his resignation .

As political analysts warned the row would fuel support for the far right Front National, the FN's president, Marine Le Pen, told French television: "The UMP no longer exists. The UMP is finished.

"Whoever runs the UMP will have no legitimacy. Either they re-run the election, which would be reasonable, or the UMP announces its death and at that moment becomes two structures."

In the vacuum left by the implosion of the official opposition party, the FN has been quick to vaunt itself as the only party able to take on François Hollande's Socialist government.

"We will welcome [UMP members] with open arms, because the real battle against the left must be fought and today, tomorrow and in the months to come the UMP is clearly not in a state to fight it," she said.