Francois Fillon has refused to step down from the French presidential election race in the face of mounting pressure over a “fake work” scandal involving his wife.

The 62-year-old former Prime Minister abruptly cancelled a high-profile appearance on Wednesday, before giving a speech at his campaign headquarters in which he revealed he has been summoned for questioning over “Penelopegate” on 15 March.

Mr Fillon said he continues to reject all corruption allegations, calling the investigation process unfair and saying it amounted to an orchestrated “political assassination”.

“I will answer the summons, I will respect the judges…although what we have seen is not natural,” Mr Fillon said.

“I will not surrender, I will not give up, I will not withdraw, I will continue to the end because it is democracy that is under attack.”

In a bullish speech, he said he would leave the decision on his candidacy to “the French people”.

President Francois Hollande defended France's judicial system against Mr Fillon's allegations of bias.

France presidential race: Francois Fillon wins conservative candidacy

"As guarantor of the independence of the judicial authorities, I want to speak out against any calling into question of the judges," he said in a statement.

"Being a presidential candidate doesn't authorise you to cast suspicion on the work of police and judges ... or to make extremely serious accusations against the justice system and our institutions more broadly."

Mr Fillon's defiant news conference came after hours of mounting speculation that he would finally cave into pressure from within his own party to step down.

The politician has already gone back on his own pledge to remove himself from the presidential race in the event of a formal probe.

He was considered the front runner to become president until the “Penelopegate” scandal broke in January, with the former Prime Minister sliding down the polls with the launch of a judicial investigation.

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The centre-right candidate is accused of paying his wife and two of his children large sums of money for work they did not carry out.

Mr Fillon has denied wrongdoing, saying his wife was paid hundreds of thousands of euros for genuine work as his parliamentary assistant, though he has acknowledged giving her the work was an error of judgement.

After a preliminary investigation opened on 25 January, the financial prosecutor's office decided to launch a formal judicial inquiry on Friday, turning it over to investigating judges who can bring charges or throw the case out.

The list of potential charges include misappropriation of public funds, abuse of public funds and influence trafficking.

The French election has been dogged by scandals and inquiries, with leading independent candidate Emmanuel Macron forced to deny rumours of a gay extramarital affair and a separate investigation underway into spending by Marine Le Pen.

The Front National leader has refused to be questioned over allegations she illegally used EU funds to pay parliamentary funds until after the election.

Opinion polls currently have Mr Fillon lagging in third place for the first round of the French vote, behind Ms Le Pen and the centre-left candidate Mr Macron.

The two front-runners are just a handful of percentage points apart in polls, with most putting Ms Le Pen narrowly ahead in the first round but predicting a heavy defeat to Mr Macron in the second and decisive vote.

A new Opinionway poll released on Wednesday forecast that Ms Le Pen will win the first round of voting by 25 per cent, followed by Mr Macron on 24 per cent and Mr Fillon on 21.

In the event of Mr Fillon reaching the second round, he would beat Ms Le Pen by 60 per cent to 40 per cent of votes.