A French judge is to question former French president Nicolas Sarkozy over allegations of illegal donations to his UMP party, it was announced on Monday, as the party collapsed into bitter recriminations after a membership vote for a new leader and squabbling.

Sarkozy has been summoned by a judge in Bordeaux investigating claims that he received money for his successful 2007 presidential election campaign from France's richest woman, Liliane Bettencourt.

The former leader has always denied the allegations for which his campaign treasurer has been put under formal investigation.

Le Figaro newspaper said Sarkozy would appear before the judge on Thursday. He lost his legal immunity when he lost the presidential election to the Socialist François Hollande in May.

The report came as the party failed to elect a new leader after a membership vote on Sunday and imploded into a pool of vitriol. Both candidates claimed victory and supporters from the rival camps accused each other of cheating.

As the party's internal electoral commission counted and recounted the votes during the day, appeals for calm were drowned out by waves of accusation and counter-accusation.

Political commentators described the situation as "surreal". Others spoke of madness and ridicule, while party grandees admitted, with considerable understatement, that the fiasco was "extremely damaging" to the party. Former foreign minister Alain Juppé, one of the UMP's founder members 10 years ago, issued an "alarm warning".

"What is happening is even worse than I foresaw," he told French TV: "The very existence of the UMP is at risk today."

The row left leaders of the far-right National Front, which accuses the UMP of stealing its policies and voters, rubbing their hands with glee. The FN, led by Marine Le Pen, is now the third force in French politics, and might gain from a collapse of the mainstream right.

"It is not an unpleasant situation for us," said Louis Aliot of the FN. Another FN official described the UMP election as a "car crash".

The UMP's 300,000 members had been asked to decide between the hawkish Jean-François Copé, 48, and former prime minister François Fillon, 58. Copé courted the party's right wing by vaunting the merits of an "uninhibited" UMP addressing subjects such as "anti-white racism". Fillon sold himself as more moderate, unifying and conciliatory, and had led the popularity polls.

In an increasingly bitter campaign, both Copé and Fillon tried to maintain a veneer of civilised politeness. But as the weeks wore on it became clear there was little love lost between them.

Copé astonished political commentators on Sunday evening by going on television to declare he had won, a move described by critics as a "media coup". France-Inter radio likened his behaviour to that of a "South American colonel from the 1960s". Fillon retaliated by furiously insisting he was the winner.

On Monday there were calls for calm from party heavyweights who appealed for rival camps to stop the "invective".

Valérie Pécresse, a former government minister and supporter of Fillon, told Europe 1 the situation was "ridiculous". "I fail to understand why Mr Copé leaped forward to announce a victory, when this is not corroborated by any official figure given by the commission," she said.

One frustrated UMP member told French radio: "Neither Fillon nor Copé has won … we can only hope Nicolas Sarkozy will make a return."