By James Mackenzie

PARIS, May 15 (Reuters) French Prime Minister Dominique de Villepin faces a censure vote in parliament tomorrow as he battles to survive a smear scandal that has engulfed his centre-right government and threatened his own political future.

The size of the government's majority means the motion brought by the opposition Socialists is almost certain to fail.

But the weekend decision by Francois Bayrou, head of the centrist UDF party -- usually allied to the right -- to back the motion has underlined the fragility of Villepin's position.

''The suicide of the right,'' declared the daily Le Parisien after Bayrou's decision, which was illustrated by the left-leaning daily Liberation with a cartoon showing him as a TNT-strapped suicide bomber.

Bayrou's move has added a further twist to a murky saga that has snared France's political leaders for weeks.

The governing centre-right UMP controls 364 votes out of 577 in the National Assembly, enough to ensure the survival of the government. Still, the tone of the debate will be closely watched for any signs of flagging support.

Villepin has been accused of using French spies to try to smear his own Interior Minister Nicolas Sarkozy, a likely rival in next year's presidential election, in a scandal that has exposed a state of virtual warfare within the government.

President Jacques Chirac, who has strained relations with Sarkozy, has stood by Villepin. But his own credibility has been badly damaged by the ''Clearstream affair'', named after the Luxembourg bank where Sarkozy was wrongly suspected of holding secret accounts.

''NEST OF VIPERS'' Tomorrow's vote will be the third faced by Villepin's government in the past year.

Bayrou has said his decision does not mean all 30 UDF deputies will join him to criticise what he calls a ''nest of vipers'' at the top of government. But he told Liberation: ''It is impossible for me to accept the situation.'' After apparently hesitating over whether to remain in government or resign, Sarkozy, who is president of the UMP, made clear at the weekend he would stay on as Interior Minister, preventing an outright government breakdown.

Socialist Party leader Francois Hollande said the decision by the UDF leader to support the censure vote meant that the governing coalition had effectively fallen apart.

''I don't consider that Francois Bayrou is in the left wing opposition but I do think there isn't a majority any more,'' he told the Le Monde daily.

Villepin has strongly denied asking French intelligence officials to try to dig up dirt on Sarkozy after his name appeared on a fake list apparently showing financial transfers in Luxembourg linked to a controversial arms sale in 1991.

But his chances of succeeding Chirac as president appear to have been severely damaged by the fallout from the affair.

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