'Penelopegate': the Achilles' heel of France's Fillon

French right-wing candidate for the upcoming presidential election Francois Fillon (R) flanked by his wife Penelope (L), looks on during a campaign rally in Paris in January 2017

Francois Fillon campaigned as a sleaze-free reformer, but his bid for the French presidency appears on the brink of collapse over claims he used taxpayers' money to pay his wife Penelope for fake jobs.

- Who is Francois Fillon? -

Fillon, who turns 63 on Saturday, was prime minister from 2007 to 2012, the high point of a political career spanning nearly four decades.

The staunch Catholic emerged in November as the surprise nominee of the conservative Republicans party after promising to slash public spending and cut bureaucracy. He notably also campaigned on his reputation for integrity.

Before the expenses scandal, voter surveys consistently showed him as the likely winner of the two-round presidential election in April and May.

- What is he accused of? -

The satirical and investigative weekly Canard Enchaine revealed in January that Fillon had put the Welsh-born Penelope and two of their children on the public payroll, earning the family nearly 900,000 euros ($950,000) before taxes.

The paper said Penelope earned around 700,000 euros over 15 years as a parliamentary assistant despite scant evidence that she did any work. The press has dubbed the scandal "Penelopegate".

Their children Marie and Charles also earned a total of 84,000 euros as parliamentary assistants in 2005-07.

- Did Fillon do anything illegal? -

Fillon announced Wednesday that he is to be charged on March 15. Prosecutors have said he is under investigation for possible embezzlement of public funds.

At a defiant press conference on Wednesday, Fillon vowed that he could prove his innocence, saying: "I won't give in, I won't surrender and I won't withdraw."

Employing a family member as a parliamentary aide is common in France and not illegal, unlike in Germany or at the European Parliament.

But there are suspicions that Penelope did no work for her parliamentary salary, which exceeded 10,000 euros pre-tax a month in 2007. She had neither a security pass for the parliament building nor a work email account.

Moreover, she said in a newspaper interview that she was "never involved in my husband's political life".

She presented herself as a low-key political wife who preferred a quiet life at the family's country manor house near Le Mans in northwest France to mingling with Parisian politicos.

- How did Fillon handle the scandal? -

Fillon apologised early last month for having hired Penelope, saying he deeply regretted the "error". But he repeatedly refused to step aside, saying grassroots support was still behind him.

After previously promising to quit if he was charged with criminal offences, on Wednesday he changed tack, angrily denouncing the case as an attempted "political assassination".

- Who stands to gain from his exit? -

Scores of allies have turned against Fillon, and veteran politician Alain Juppe, 71, is at the ready to take over as the conservative standardbearer if he throws in the towel.

Fillon's spokesman Thierry Solere on Friday became the most high-profile aide to abandon the campaign after two deputy directors, the treasurer and his foreign affairs point man.

Centrist candidate Emmanuel Macron, a 39-year-old former economy minister, has been buoyed in the polls since the scandal broke.

He has capitalised on it heavily, unveiling a string of policies to clean up French politics including a ban on MPs employing family members.

In first-round voter intentions, Fillon is trailing behind Macron and far-right leader Marine Le Pen -- whose support has held even though she is caught up in an expenses scandal of her own.

The avuncular Juppe, for his part, would immediately take a slight lead in the race, in what would be yet another dramatic twist in a rollercoaster election, a poll published Friday showed.