The decision is likely to deepen a growing rift between Sharif, who is the country's most popular politician, and the pro-Western government under President Asif Ali Zardari.

"Asif Ali Zardari had a hand in the disqualification of Nawaz Sharif, and today's decision is also according to his wishes," Akram Sheikh, a lawyer for Sharif, told reporters after the verdict.

Zardari's year-old coalition government is not directly threatened by Sharif's increasingly vocal opposition.

However, the ruling could trigger a destabilizing power struggle in the country's richest and most populous province, Punjab, where Sharif's party currently governs in coalition with that of Zardari.

Top Pakistani officials, including the foreign minister and army chief, are currently in Washington as part of the Obama administration's efforts to draw up a new strategy for Pakistan and Afghanistan.

Presiding judge Musa Leghari dismissed appeals against a ruling in a lower court that disqualified Sharif from contesting Pakistan's 2008 elections because of a prior conviction.

It also upheld a challenge against the election of Sharif's brother, Shahbaz Sharif, to a seat in the Punjab Assembly. That means Shahbaz Sharif cannot continue as head of the provincial government.

The brothers appear unlikely to ask the Supreme Court to review its decision. They have refused to appear before it in person and cast doubt on its legitimacy to hear the case.

Sharif has already announced his support for a massive rally next month by lawyers whose protests over the past two years helped drive former President Pervez Musharraf from power.

The Sharif brothers did not immediately say whether they will move the Supreme Court to review its judgement, but they have refused to appear before the court in person, casting doubts on its legitimacy to hear the case.

The main opposition PML-N reacted angrily to the judgement, with party spokesman Siddique-ul-Farooq saying: "We do not recognise these courts as genuine and constitutional courts. These are kangaroo courts and we care a fig for this judgement. It has no value."

Farooq said the judgement also proved Sharif's allegation that Zardari was conspiring to get the Sharif brothers disqualified from contesting polls through "kangaroo courts".

Senior PML-N leader Khawaja Asif, a close aide of Sharif, said: "some people in the Pakistan People's Party have chosen to draw the battle lines" and the PML-N would have "no option but to fight back".