PARIS (Reuters) - France's ex-president Nicolas Sarkozy and his camp on Wednesday stepped up attacks on his rival Alain Juppe and a centrist ally they said could not be trusted, in a sign of growing tensions within their conservative Les Republicains (LR) party.

Sarkozy is lagging in opinion polls behind Juppe ahead of a Nov. 20 first round of voting in a primary contest that will decide who wins the LR nomination for next year's French presidential election.

Sarkozy is unhappy about a rule that allows anybody willing to pay two euros and sign a declaration that they share center-right values to take part in the primaries, which many see as effectively a first round of the presidential election itself.

The rule is widely viewed as favoring Juppe, a former prime minister, as the much more abrasive Sarkozy is unpopular among left-wing voters. Many such voters have told pollsters they will take part in the primaries to back Juppe and block Sarkozy.

Sarkozy, who served as president from 2007 to 2012, said on Wednesday Juppe was making an "error" by accepting the backing of Francois Bayrou, head of the small center-right MoDem party.

"I don't want the next government to be Bayrou's hostage," he told France Info radio.

Bayrou voted in the 2012 presidential election for Francois Hollande, the Socialist candidate who defeated Sarkozy. Sarkozy says Bayrou cannot be trusted and would prevent Juppe carrying out key reforms.

Several of Sarkozy's closest allies also slammed Bayrou on Wednesday at a joint news conference. Lawmaker Luc Chatel said "the ambiguity" of Bayrou's support for Juppe could call into question the party's "founding pact".

While Sarkozy, like Juppe and the other five, lesser known candidates in the primaries, have said they will respect the result of the primaries, analysts say the current tensions could spell trouble for the LR party.

"The Sarkozy camp is insisting on this (aggressive stance) to try and deprive Alain Juppe of his legitimacy because Bayrou voted for Hollande," said Jerome Fourquet at Ifop pollsters.

"We're seeing the start of a discourse on an illegitimate victory, that will take its toll," Fourquet said, adding that in case of a narrow Juppe win where left-wing and Bayrou supporters make the difference, "many grassroot (LR) party members will be very angry".

An Elabe poll on Wednesday forecast that Juppe would win the run-off of the primary vote with 61 percent - up five percentage points since early October - against 39 percent for Sarkozy.

The winner of the primaries has a strong chance of becoming France's next president, given the deep unpopularity of Hollande and the fact that a majority of voters do not want Marine Le Pen, leader of the far-right National Front, as president.

(Reporting by Ingrid Melander and Sophie Louet; Editing by Gareth Jones)