The Ukrainian Supreme Court today rejected as invalid the official results of last month's presidential run-off election and ruled that a new head-to-head vote must be held.

The court's decision was a major victory for Viktor Yushchenko, whose complaint of massive electoral fraud has been upheld. The opposition leader will be clear favourite to win the re-vote, which must be held by December 26.

Tens of thousands of opposition protesters who had massed in Kiev in anticipation of the decision cheered, waving blue-and-yellow Ukrainian and orange Yushchenko flags. The crackle of fireworks could be heard in the courtroom from the central Independence Square where the protesters have gathered over the past three weeks.

The court issued its verdict in response to an appeal by Mr Yushchenko, who asked it to cancel the official results of the November 21 run-off, which he said had been stolen by Viktor Yanukovych, the Russian-backed Prime Minister.

Leonid Kuchma, the outgoing President who annointed Mr Yanukovych as his successor, had argued that a re-run of the second round of voting would be unconstitutional, and an entirely new election should be held. That suggestion has been rejected by Mr Yushchenko.

But the court said that its decision was final. "The court decision is final and cannot be appealed," Anatoly Yarema, the presiding judge, said.

Jeremy Page, the Times correspondent in Kiev, said: "It's a huge victory for Yushchenko. The Supreme Court has backed up his claim that the Government rigged the election.

"It's also come at a crucial time for the opposition - their 'revolution' was starting to flag and lose momentum a bit with all the legal wrangling.

"The only sting in the tail is that Parliament will push through reforms to have a more balanced system, with a parliamentary government, not the top-heavy system they have now where the president chooses the prime ministers. So constitutionally, he won't have as much power as he would have liked."

There was no immediate reaction to the decision from the Yanukovych camp, but an opposition MP, Mykola Katerinchuk, said: "This is a great victory for all the people in the square. It's a great victory for democracy."

Adrian Karatnycky, senior scholar at at US-funded democracy advocates Freedom House, said: "This is the birth of Ukrainian democracy and a victory for the rule of law. This is the end of Russian aspirations for hegemony."

The crisis has strained ties between Russia, concerned about losing influence in what it regards as its backyard, and the West, keen to see a stable democracy on the edge of an expanded European Union.

Mr Kuchma flew yesterday to Russia to meet President Putin, who has publicly backed him and Mr Yanukovych. That underlined Kremlin fears that if Mr Yushchenko took power he would weaken links with Moscow and push Ukraine deeper into the West's embrace.

Mr Putin said: "A re-vote could be conducted a third, a fourth, a 25th time, until one side gets the results it needs. It would yield nothing."

For Mr Kuchma the court's ruling is more than just a political defeat. His ten years in office were tainted by scandal and he has no automatic immunity from prosecution once he steps down.

Mr Yanukovych, who has cut an increasingly isolated figure in recent days and been taken ill, earlier said only a new election -- rather than a re-run -- would solve the crisis.

"We need the certainty that no one will question the new vote," he told Italy's La Repubblica daily. "We can only restart with a clean slate."

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