Jerusalem (AFP) - Israel's top court on Tuesday upheld a bribery conviction against former premier Ehud Olmert, which will make him the country's first ex-prime minister to serve jail time.

The 70-year-old, who was prime minister from 2006 to 2009, was sentenced in May last year to six years in prison on two separate charges of taking bribes in the early 2000s.

The Supreme Court on Tuesday reduced his sentence to 18 months in prison, and acquitted him of one of the charges.

"Olmert has been acquitted of the felony of taking a bribe of 500,000 shekels ($128,500/117,000 euros)," the ruling said, but "unanimously convicted of the felony of taking a bribe of 60,000 shekels".

The prison term will begin on February 15, it said.

The trial of the former premier was linked to the construction of Jerusalem's massive Holyland residential complex when Olmert served as the city's mayor.

The towering construction project, which dominates the city's skyline, is seen as a major blot on the landscape and widely reviled as a symbol of high-level corruption.

The 500,000 shekel bribe was linked to the Holyland affair, while the 60,000 bribe was related to election campaign funding.

"A heavy weight was lifted from my chest today, when the Supreme Court exonerated me of the main charge, of Holyland," a tired-looking Olmert told reporters outside the courtroom.

"I was never offered bribes, nor did I ever take any," he said.

- 'Nobody above the law' -

The court also issued rulings on the appeals of seven other men sentenced last year, including former Jerusalem mayor Uri Lupolianski, whose six-year sentence for bribery was converted to six months of community service due to his poor health.

Isaac Herzog, head of the opposition Labour party, said the outcome "proved that nobody, not even a prime minister, is above the law."

"May this serve as a lesson to public figures -- never use the power given to us... for personal gains," he said in remarks relayed by a spokesman.

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It is not the first time a former Israeli leader has been jailed -- ex-president Moshe Katzav is serving a seven-year prison term for rape and sexual harassment.

Before his own incarceration, Olmert will be in the Supreme Court on January 19 for an appeal in a separate case.

The Jerusalem District Court in May sentenced Olmert to eight months in prison for fraud and corruption following a retrial over allegations that he had received envelopes of cash from a US businessman while trade and industry minister.

Born near the port city of Haifa, Olmert was elected to parliament in 1973 as a member of the rightwing Likud party and was mayor of Jerusalem from 1993 to 2003.

He later served as a cabinet minister, holding the trade and industry portfolio as well as several others.

He broke off from Likud with then-prime minister Ariel Sharon to form centre-right Kadima in 2005 and became premier in 2006 after Sharon slipped into a coma.

Olmert later resigned after police recommended that he be indicted in several graft cases.

Gerald Steinberg, professor of political science at Bar Ilan University near Tel Aviv, said for most Israelis the case would be Olmert's "defining legacy" that could prove a lesson to aspiring younger politicians.

"The next generation sees they can't just get away with it," he told AFP, adding that the public is far less trusting of politicians than previously.

"There is less naivety and more (public) concern that there be checks and balances on decisions after Olmert," he said.

Israeli legal affairs commentator Yonah Jeremy Bob, however, warned that Olmert's acquittal of the Holyland-linked charge may actually make it harder to get convictions of high-profile figures.

Writing in The Jerusalem Post, Bob said the judges' decision to effectively throw out the evidence of the prosecution's chief witness -- who died mid-trial -- would raise the level of evidence needed in future cases.