Ukraine's Supreme Court has agreed to hear an appeal against last month's presidential election result by the runner-up, Viktor Yanukovych.
Russia's President Putin wanted Viktor Yanukovych to win
The court has already rejected several appeals from Mr Yanukovych's team over the re-run election, won by pro-Western opposition leader Viktor Yushchenko.
The voluminous appeal lodged on Friday - and delivered in a minivan - urged the court to order a re-vote.
Mr Yushchenko cannot be sworn in until the court rules on all complaints.
The official results show that Mr Yushchenko won by two million votes, with 51.99% of the vote. Mr Yanukovych won 44.2%.
Outgoing President Leonid Kuchma has already accepted Mr Yanukovych's resignation as prime minister.
Mr Yanukovych, who had been in the job since November 2002, stepped down on 31 December.
The presidential poll was re-run on 26 December after Mr Yushchenko and independent observers alleged extensive vote-rigging in the original poll, which Mr Yanukovych was said to have won. The Supreme Court upheld their complaints of massive fraud.
Independent observers said the re-run was much fairer.
More than 600 volumes of evidence were delivered to the Supreme Court to support Mr Yanukovych's appeal on Friday. His team claims the rights of millions of voters were violated.
But Mr Yanukovych has previously admitted that he is not optimistic about the outcome of the appeal.