French politician Francois Fillon (R), member of the conservative Les Republicains political party, arrives after partial results in the first round of the French center-right presidential primary election at his headquarters in Paris, France, November 20, 2016. REUTERS/Gonzalo Fuentes TPX IMAGES OF THE DAY

By Ingrid Melander and Michel Rose

PARIS (Reuters) - French ex-president Nicolas Sarkozy on Sunday conceded defeat in the race to be the conservatives' nominee for next year's presidential election after his former prime minister, Francois Fillon, stormed to a winning lead in the primaries.

Sarkozy, president from 2007-12, said he would now back Fillon in an expected runoff against Alain Juppe, another former prime minister, on Nov. 27 in a contest which is likely to produce France's next president in May.

According to results based on 8,709 polling stations out of a total 10,229, Fillon was seen gathering 44.1 percent of the votes, Juppe 28.2 percent and Sarkozy 21.0 percent, with the gap with Sarkozy widening from the first partial results.

"I failed to convince a majority of voters," Sarkozy told supporters and reporters at his campaign headquarters.

"I hold Alain Juppe in high esteem, but I feel closer to Francois Fillon's political choices," he said, indicating he would vote for his former prime minister.

Fillon, 62, long considered as a political has-been and trailing Juppe and Sarkozy in opinion polls, was the unexpected frontrunner of Sunday's vote. An admirer of late British prime Minister Margaret Thatcher, he is a rare economic liberal in largely statist France.

Fillon also received the support of Bruno Le Maire, seen as having come fourth or fifth in the vote.

The most centrist of the seven contenders in the primaries, Nathalie Kosciusko-Morizet, who was fighting Le Maire for the fourth spot, threw her weight behind the 71-year-old Juppe.

Socialist President Francois Hollande, who is deeply unpopular, has yet to announce whether he himself will stand again.

With the Left very divided and a majority of voters seen in opinion polls to be opposed to seeing the far-right National Front in power, the chosen centre-right nominee is likely to defeat party leader Marine Le Pen in an expected election run-off next May.

But while polls have consistently shown Juppe would easily beat Le Pen, there have been no recent surveys on how Fillon would fare in such a match, in further evidence of how unexpected his top spot on Sunday was.

Polls have shown that Fillon, who had received backing by opponents of France's gay marriage laws, is much less popular than Juppe amid left-wing voters, which could make it harder for him to get their vote versus Le Pen.

Until a week ago, Fillon, a social conservative with economically liberal ideas, trailed Juppe and Sarkozy badly in polls and had not been expected to go through to round 2 of the primaries.

Juppe, a moderate conservative campaigning on an inclusive, "happy identity" platform, had for months been ahead in polls.

But over the past week the contest has been transformed into a tight race between the three men.

Sarkozy has sought to tap into populist sentiment while Fillon is proposing tough measures to shake up the economy.

(Additional reporting by Richard Lough, Andrew Callus, Marine Pennetier, Sophie Louet and Simon Carraud; Writing by Ingrid Melander; Editing by Richard Balmforth)