Israeli Prime Minister Ehud Olmert is facing mounting calls to resign over a criminal probe into allegations he took bribes from a millionaire US financier.

Olmert has vehemently denied any wrongdoing but has said he would quit if he is charged in a case that threatens to shake the political landscape at a crucial moment in Middle East peacemaking.

Pressure on the 62-year-old premier mounted after a gag order over the case was lifted on Thursday. The timing was particularly embarrassing for Olmert, coming as Israel celebrated its 60th anniversary and a week before a scheduled visit by US President George W. Bush.

Gideon Star, who heads the parliamentary group of the conservative opposition party Likud, said that "considering the seriousness of the suspicions that surround Olmert, he is no longer in a position to carry out his duties".

Olmert has been dogged by scandals since he took office in 2006 and even his coalition partners are now getting edgy.

"It's the straw that broke the camel's back, considering all the previous investigations," said Eytan Cabel, secretary general of the Labour party said on Friday.

Meanwhile, Palestinian officials expressed fears the affair could affect the peace process, particularly if early elections are called.

"In the immediate future, we fear this crisis could have an impact on talks led through Egypt's mediation for a truce (in the Gaza Strip)," said Palestinian negotiator Saeb Erakat.

"In case of early elections, the peace process will be put on hold," Erakat told AFP.

White House Spokesman Gordon Johndroe stressed that Bush will go to Israel as planned and will meet with Olmert.

The president is pushing for an Israeli-Palestinian peace deal to be reached before he leaves office in January even though little progress has been made since long-dormant negotiations were restarted in November.

The justice ministry said on Thursday that Olmert was being investigated over suspicions he unlawfully received payments from a foreign businessman during his time as mayor of Jerusalem and as industry minister.

"Citizens of Israel, I look you in the eye and I say to you, in no uncertain terms, I have never taken a bribe, nor have I unlawfully pocketed money," Olmert said at a hastily convened press conference on Thursday.

"If the attorney general decides to file an indictment against me I shall resign immediately, even though I am not required to do so by law."

Olmert acknowledged that he had received financial contributions for various election campaigns from Jewish-American businessman Morris Talansky, 75, but insisted they were not illegal.

Anti-fraud investigators had grilled Olmert for an hour on Friday, while his former office manager, Shula Zaken, has been questioned four times.

Olmert took office after his predecessor Ariel Sharon collapsed into a coma in January 2006 and then led his centrist Kadima party to election victory in March of that year.

He has weathered a string of corruption scandals, massive unpopularity, accusations of failings in the 2006 war against Hezbollah and a cancer scare.

Before the latest case came to light, Olmert was already the subject of three police inquiries into suspected corruption involving potential conflicts of interest, fraudulent property transactions and abuse of power in connection with political appointments. Another case against him was dismissed.

When a premier resigns it is up to the president, currently Shimon Peres, to pick a successor who can try to form a government. Failing that, early elections must be held.

Foreign Minister Tzipi Livni, a member of Kadima, has been tipped as the most likely candidate to succeed Olmert if he goes.

Former prime ministers Ariel Sharon, Ehud Barak and Benjamin Netanyahu also faced corruption investigations, none of which ever led to indictments.

Then Israeli president Moshe Katsav last year became the country's most senior official to step down over wrongdoing after he was found guilty of sexual harassment.