BUCHAREST (Reuters) - Romania's centrist opposition is filing a no-confidence motion against the government over a decree to decriminalize certain graft offences, the leader of the National Liberal Party (PNL) said on Wednesday.

"We file it in the next hour, we gathered the needed number of signatures to back it, together with the USR (Save Romania Union," PNL leader Raluca Turcan told Reuters by telephone.

"The motion is headlined: "Do not legalise theft in Romania."

The Social Democrat-led government of Prime Minister Sorin Grindeanu, which holds a comfortable majority in parliament, is expected to survive the vote, due to be hold sometime next week.

Earlier in the day, the country's top judicial watchdog announced a court challenge against the decree that critics say is the biggest retreat on reforms since the country joined the European Union a decade ago.

(Reporting by Radu Marinas; Editing by Toby Chopra)