A Ukrainian court says a key change to the country's election law violates the constitution, in a dramatic ruling on the eve of Sunday's presidential vote.
The ruling means disabled voters will not have to trek to the polls
A reform limiting voting from home was passed by parliament amid the crisis generated by November's disputed poll.
Viktor Yanukovych's backers challenged the law, saying it would discriminate against the disabled and housebound.
He faces Viktor Yushchenko in a run-off vote for president on Sunday after the first result was annulled due to fraud.
A package of reforms was brought in by parliament on 8 December as part of a compromise worked out between the Yushchenko camp and outgoing President Leonid Kuchma.
The measures were designed to resolve the crisis generated by the disputed presidential election, awarded to Mr Yanukovych, that brought hundreds of thousands of protesters out on to the streets.
The Constitutional Court has now annulled the restrictions on home voting but let the other reforms stand.
31 October: First round of voting gives Mr Yushchenko slight lead
22 November: Second round gives Mr Yanukovych lead, but opposition disputes result
3 December: Supreme Court annuls result
24 December: Campaigning ends
26 December: Re-run of second round due
Analysts say the ruling makes it far less likely that the vote result can be challenged on constitutional grounds by either side.
The head of the election commission, Yaroslav Davydovych, said Sunday's vote would go ahead, taking into account the court decision.
"We don't have another alternative. The vote must be held," he said.
Under the ruling, election officials will have to accept ballots from voters who register to say they are unable to go to polling stations.
Correspondents say the so-called "home voting" was one of the key areas of contention between the two sides, with accusations that it was used to perpetrate fraud through the stuffing of ballot boxes.
About 12,000 international observers are making final preparations to monitor the election and try to ensure that any violations are prevented.
Some observers have said that overturning any of the reforms would throw Sunday's election into doubt - a message that had been echoed by the Yushchenko camp.
But a spokeswoman for Mr Yushchenko, Irina Gerashchenko, told reporters that the opposition's chances would not be harmed.
"The decision of the court will not disrupt the election," she told reporters.
Nestor Shufrych, a legislator and Yanukovych ally said the court's ruling would affect about three million people. He said it could yet be a basis for another election challenge.
"Of course, if one side or another is not pleased with the results of the vote, they will appeal on the basis of the Constitutional Court's decision," Mr Shufrych was quoted as saying by the Associated Press news agency.
Exit polls indicated that Mr Yushchenko had won the 22 November vote, but Mr Yanukovych was declared victorious before the Supreme Court annulled the result because of fraud concerns and ordered a re-run.