France's new law punishing denial of the Armenian genocide was put on hold after politicians opposed to the legislation demanded that its constitutionality be examined.
Turkey reacted furiously last week when the Senate approved the law which threatens with jail anyone in France who denies that the 1915 massacre of Armenians by Ottoman Turk forces amounted to genocide.
President Nicolas Sarkozy's office brushed off angry threats of retaliation by Turkey and vowed to enforce the law within a fortnight.
But on Tuesday two separate groups of French politicians who oppose the legislation -- from both the Senate and the lower house of parliament -- said they had formally requested the constitutional council to examine the law.
The groups said they each had gathered more than the minimum 60 signatures required to ask the council to test the law's constitutionality.
"This is an atomic bomb for the Elysee (Sarkozy's office) which didn't see it coming," said deputy Lionel Tardy, who said that most of the 65 signatories from the lower house were, like him, from Sarkozy's UMP party.
The council is obliged to deliver its judgement within a month, but this can be reduced to eight days if the government deems the matter urgent.
Turkey's President Abdullah Gul and Prime Minister Recep Tayyip Erdogan immediately welcomed the development.
"I hope the constitutional council will do what is necessary," said Erdogan, while Gul said he was "not expecting the French from the very beginning to let their country be overshadowed" by the genocide law.
France has already officially recognised the killings as a genocide, but the new law would go further by punishing anyone who denies this with up to a year in jail and a fine of 45,000 euros ($57,000).
Armenians say up to 1.5 million of their forebears were killed in 1915 and 1916 by the forces of Turkey's former Ottoman Empire.
Turkey disputes the figure, arguing that 500,000 died, and denies this was genocide, ascribing the toll to fighting and starvation during World War I and accusing the Armenians of siding with Russian invaders.
Erdogan last week denounced the law as "tantamount to discrimination and racism" and warned that his Islamist-rooted government would punish Paris with unspecified retaliatory measures if Sarkozy signed it into law.
Ankara has already halted political and military cooperation with France and was threatening to cut off economic and cultural ties.
Trade between the two states was worth 12 billion euros (ê15.5 billion) in 2010, with several hundred French businesses operating in Turkey.
Armenia hailed the passage of the bill through the French Senate, with President Serzh Sarkisian writing in a letter to Sarkozy: "France has reaffirmed its greatness and power, its devotion to universal human values."
Around 20 countries have officially recognised the killings as genocide.
Amnesty International has criticised the French law, saying it would violate freedom of expression.