PARIS - French presidential candidate Francois Fillon announced Wednesday that judges had summoned him to press charges over an expenses scandal but vowed to continue his campaign.

The 62-year-old conservative former premier was favourite at the start of the year to win the French presidency after clinching the nomination for the Republicans party in November. But he has since been hit by a series of allegations that he paid his British-born wife Penelope and his children hundreds of thousands of euros since 1988 for allegedly fake parliamentary jobs. Fillon said the charges were "entirely calculated to stop me being a candidate for the presidential election."

 "I won't give in, I won't surrender and I won't withdraw," he told a press conference on Wednesday. It is the latest twist in an extraordinary campaign that has regularly wrong-footed observers ahead of the two-stage contest on April 23 and May 7.

Last week, French prosecutors launched a full judicial inquiry into Fillon's use of his parliamentary budget, increasing pressure on his campaign which has been in crisis since allegations first surfaced in January.

He has been summoned to be charged on March 15.

Fillon has been defiant throughout the affair, insisting publicly that the facts would exonerate him and reminding his party that they have few attractive alternatives to replace him.

He said Wednesday he was the victim of a "political assassination" and has accused the media of trying to "lynch" him.

He has pointed the finger at President Francois Hollande and the socialist government, which he believes has encouraged the investigations to discredit him.

Recent surveys suggest that far-right leader Marine Le Pen and centrist candidate Emmanuel Macron are the two most likely to progress from the first round of the election on April 23.

Macron, a 39-year-old who only launched his political movement "En Marche" ("On the Move") last April, is currently shown as the winner of a run-off vote on May 7.

He urged the justice system to be "allowed to do its work as normal" while reminding Fillon that victory in the election would not mean he was cleared.

The allegations against Fillon are particularly damaging after he campaigned as a sleaze-free reformer ready to administer a "radical" economic overhaul and cut wasteful public spending.

He has also had to backtrack on a previous pledge to withdraw if ever he was charged.

"I submit myself before the French people, because only the ballot box, and not an investigation directed against me, can decide who will be the next president of the republic," he said on Wednesday.

Fillon had postponed a visit to a key farm show earlier in the morning and announced the press conference, sending the rumour mill into overdrive amid speculation he would withdraw.

Though employing a family member is not illegal in France, Fillon has not published evidence publicly of Penelope's work beyond her contracts which show various periods of employment since 1988.

French media have also focused on previous interviews given by Penelope.

"Until now, I have never got involved in my husband's political life," she told regional newspaper Le Bien Public at the end of last year, echoing a similar statement to Britain's Sunday Telegraph newspaper in 2007.

After five years as a loyal prime minister to ex-president Nicolas Sarkozy from 2007-2012, Fillon emerged from his shadows during the campaign for the Republicans presidential nomination at the end of last year.

British-born Penelope, or Penny as she is known, and two of their five children were paid around 900,000 euros ($950,000) from public funds from Fillon's office budget, the Canard Enchaine newspaper has reported.

The couple met aged 23 and were married three years later. Their main home is a 12th-century manor near the western city of Le Mans.

In an unusual twist, Fillon's brother is married to Penelope's sister.