The British-born wife of French presidential candidate François Fillon has been formally put under investigation in the fake jobs scandal that has poisoned her husband’s political career.

Penelope Fillon is being prosecuted for embezzlement, misappropriation of public funds and aggravated fraud, it was reported late on Tuesday evening.

The announcement that she had been mise en examen – the equivalent in French law of being charged or arraigned – came after several hours of questioning by the French financial prosecutor.

Her husband was put under investigation two weeks ago, but insisted on continuing his bid to become France’s next leader. Once considered a favourite to become president, Fillon, 63, has seen his chances seriously damaged by the fictitious jobs allegations and other scandals.

Less than four weeks from the first round vote, he is now in third place behind the far-right Front National president Marine Le Pen and independent centrist Emmanuel Macron.

A third suspect, Marc Joulaud – who stood in for Fillon in the Assemblée Nationale when the politician was made a government minister, and also reportedly employed Penelope Fillon – has also been mis en examen.

The scandal erupted in January when the satirical newspaper Le Canard Enchainé revealed that Fillon had paid his wife and two of their five children more than €900,000 (£781,000) for over 10 years to work as his parliamentary assistants. She was also allegedly paid another €100,000 to work for a literary review owned by one of her husband’s friends.

The beleaguered Fillon, representing opposition centre-right Les Républicains, was then mired in further allegations that he received €48,000 of bespoke suits from a wealthy friend, as well as a €50,000 undeclared loan. Fillon has since repaid the loan and told journalists he had given back the suits. However, the investigation has been extended into allegations that employment contracts discovered in a search of offices at the Assemblée Nationale and relating to Penelope Fillon’s work may have been altered or forged.

Fillon and his 61-year-old wife, a solicitor’s daughter from Llanover near Abergavenny in Wales, have denied any wrongdoing. The presidential candidate has accused the government of trying to discredit him, attacked the legal system and lashed out at the media.

French members of parliament are entitled to employ relatives if the jobs are real and work is done. The French financial prosecutor decided after a preliminary investigation that there was enough doubt over whether Penelope Fillon’s job was “fictitious” to continue the inquiry. The couple’s two eldest children, Marie and Charles, have also been questioned by the financial prosecutor.

The official accusations against her are complicity and concealment of embezzlement of public funds, complicity and concealment of abuse of social goods and complicity and concealment of fraud.

The charges raise the prospect that should Fillon be elected to the Elysée Palace, he would enjoy presidential immunity from prosecution during his time in office, while his wife could find herself in court.

Le Pen is also under investigation for allegedly using European parliament staff allowances to pay for FN party employees, including a personal bodyguard.

Current opinion polls suggest Le Pen and Macron will win the first round vote on 23 April and go through to a second round run-off on 7 May.