Francois FillonFrancois Fillon. Thomas Samson/Reuters
Fran├žois Fillon will stand for the French conservatives in the presidential election after claiming victory over Alain Juppe in the Republican primary on Sunday.
Partial results based on four-fifths of the primaries' polling stations showed Fillon winning by a huge margin of nearly 40 percentage points.
Fillon is set to go head-to-head with Marine Le Pen of the far-right National Front party in May's election, meaning the French left wing is set to be excluded from the contest altogether after five years of socialist Francois Hollande in power.
Fillon, a socially conservative free-market advocate who has been described as France's answer to former UK Prime Minister Margaret Thatcher, had won over 67% of the vote in a one-on-one battle with Juppe, who trailed with about 32%.
The former prime minister pledged to unite the Republicans in a victory speech he delivered Sunday evening. "I must now convince the whole country our project is the only one that can lift us up," a visibly moved Fillon said at his campaign headquarters after Juppe conceded defeat.
"My approach has been understood: France can't bear its decline," he added. "It wants truth and it wants action. I will take up an unusual challenge for France: tell the truth and completely change its software."
Marine Le PenMarine Le Pen. BBC
A representative for the National Front said the party welcomed Fillon's victory as it represented a "great" opportunity for Marine Le Pen to take control of the party's highest office. Fillon has vowed to implement a range of tough economic policies, such as slashing public spending, raising the retirement age, scrapping the 35-hour working week, and cutting back social security.
"His project is so sharply different from ours, and it is such a harsh one, he cannot get a majority of voters to back him," the National Front's Florian Philippot told Reuters. "For us, he's a great candidate (to face in the election)."
Speaking in an interview last week, Philippot described Fillon's manifesto as a "programme of chaos." He said: "It's impossible that this austerity cure does not trigger chaos."
Opinion polls have for months forecast that the center-right candidate and Le Pen would qualify for the second round of the presidential election in May and that Le Pen would then lose.
But polls, which had until just days before his victory failed to forecast Fillon's comeback, are taken with an increasingly big pinch of salt, especially after shocking results elsewhere in the west like Brexit and Donald Trump's victory in the US presidential election.
All eyes now turn to the ruling Socialist party and to whether the deeply unpopular Hollande will decide to run for the left-wing ticket in his party's primaries in January, amid signs his prime minister, Manuel Valls, is considering a bid of his own.
France, the eurozone's second-largest economy, has faced stubbornly high unemployment under Hollande, and the past two years of his term have been marked by Islamist militant attacks that have killed 230 people and focused attention on immigration and security concerns.