France's opposition leader Nicolas Sarkozy warned Tuesday there was a real risk of the far-right National Front taking power and eclipsing his more moderate party.

"A victory for the National Front (FN) at the national level is no longer hypothetical," he told members of his conservative UMP party.

The FN won the first round of a by-election in the region of Doubs in eastern France at the weekend and now faces a run-off with the Socialist Party on Sunday, putting the UMP in a bind.

It was forced into third place, knocking it out of the running, and once again raising fears that its right-wing agenda has been usurped by the more radical FN.

Sarkozy, who only recently took the reins of the UMP ahead of a possible run for the presidency in 2017, finds himself in an awkward position: should he tell UMP supporters to back their traditional Socialist enemy in order to defeat the rising threat of the FN?

In recent years, the party has side-stepped the question when it came up, telling its supporters to vote for neither.

While former president Sarkozy stressed that "we are not advising people how to vote," he did say the UMP was "asking people to take into account this dimension" that the FN could win power at a national level.

With the FN now riding high in opinion polls, parts of the UMP feel they should in exceptional circumstances throw their weight behind the Socialists to keep out the far-right.

Sarkozy's rival to become the UMP's presidential candidate, Alain Juppe, called for members to vote for their bitter Socialist rivals, saying the FN was now the party's "main political enemy."

Amid the bitter divisions in the UMP, Sarkozy said Sunday's by-election risked "exploding" the party.