Ukraine's Supreme Court has rejected all four complaints against the conduct of the presidential election lodged by Prime Minister Viktor Yanukovych.
Yanukovych refuses to step down as prime minister
The Central Election Commission also rejected his appeal over Sunday's vote, which was a re-run of the second round.
A new complaint to the Supreme Court against that decision now appears to be his last hope to overturn the result.
Mr Yanukovych lost the election to opposition leader Viktor Yushchenko, who had accused him of ballot-rigging.
Mr Yushchenko cannot be proclaimed the winner until his rival exhausts all legal possibilities to challenge the election results.
The official results showed Mr Yushchenko secured a majority of more than two million votes.
International observers praised the conduct of the re-run, saying it was much fairer than the earlier rounds.
Sunday's re-run followed a Supreme Court ruling upholding the Yushchenko camp's claim that the second round - won by Mr Yanukovych - had been massively rigged.
Mr Yushchenko's supporters thronged the streets of Kiev denouncing the election fraud.
Mr Yanukovych has defiantly refused to step down as prime minister, despite Mr Yushchenko proclaiming his government illegal.
On Wednesday, opposition supporters prevented Mr Yanukovych from entering the government compound to chair a regular cabinet meeting.
Kuchma not immune
Meanwhile, Mr Yushchenko has named possible candidates for the prime minister's post.
Speaking on Ukraine's Channel 5 TV, he named the candidates as: radical ally Yulia Tymoshenko, former Prime Minister Anatoly Kinakh, Socialist Party leader Oleksandr Moroz and businessman Petro Poroshenko.
Mr Yushchenko is aiming to form a broad reform-minded coalition cabinet.
In an interview with the UK's Guardian newspaper on Thursday, he made it clear that outgoing President Leonid Kuchma, members of his family and entourage would not be immune from prosecution under the new government.
Mr Yushchenko campaigned on an anti-corruption platform - but earlier he said that outgoing presidents should be protected from prosecution.
Yushchenko supporters blocked the government compound
He told the Guardian however that "the president has to answer under the law like any other citizen".
"Any citizen, any businessman, any politician whose actions do not correspond to national law must be punished, whether... they are the president's son-in-law or his charlady in his office."
Mr Kuchma's son-in-law Viktor Pinchuk has often been accused by the opposition of using his family connections to acquire property at rock-bottom prices.
Mr Yushchenko insisted that investigations would not be conducted "in the context of political persecution".
Touching on international affairs, Mr Yushchenko said it was time to consider withdrawing the 1,600 Ukrainian troops that form part of the US-led coalition in Iraq.
The Ukrainian contingent in Iraq operates in the Polish zone of responsibility, under Polish command.