Yulia Tymoshenko has had several rows with President Yushchenko
Aides to Ukraine's President Viktor Yushchenko have accused Prime Minister Yulia Tymoshenko of working in Russia's interests in a bid to become president.
Presidential spokesman Andriy Kyslynsky said Ms Tymoshenko had been co-operating with Russia by avoiding any criticism of its actions in Georgia.
He said she was counting on Moscow's support in next year's Ukrainian presidential election.
Ms Tymoshenko, currently on holiday, has not yet commented on the claim.
Rows between President Yushchenko and Ms Tymoshenko scuppered their previous alliance after they came to power in the Orange Revolution in December 2004.
Mr Kyslynsky said a Tymoshenko campaign headquarters had been set up in Moscow, headed by Viktor Medvedchuk. He was the chief of staff of former Ukrainian President Leonid Kuchma - the political enemy of Ms Tymoshenko and Mr Yushchenko in 2004.
"Last week, we announced that we had information about Yulia Tymoshenko's systematic work in the interests of Russia. Unfortunately, that information is now confirmed," said Mr Kyslynsky, quoted by the Interfax-Ukraine news agency.
"We will hand over the material we have to the law enforcement bodies, for detailed study," he said.
"Society has the right to get an answer to the question: How far can politicians go beyond the point where political campaigning ends and betrayal of national interests begins?"