Mr Lazarenko, who has been accused of corruption during his term of office between 1996 to 1997, was arrested as he crossed by car from France into Switzerland on Wednesday.

His arrest had been ordered by an investigating judge in Geneva.

A spokesman for the Swiss Office of Police Affairs said last month that Switzerland was helping the Ukrainian authorities with an investigation into Swiss bank accounts.

The German-language Neue Zürcher Zeitung reported in early November that bank accounts amounting to some $40m were involved.

The Office of Police Affairs says that since February 1997, it has received 20 separate requests from Ukraine for legal help in the case of Mr Lazarenko, and it has already handed over some bank documents to Ukraine.

Mr Lazarenko was prime minister in 1996-97, when he was sacked by President Leonid Kuchma.

He now leads the centrist opposition Hromoda party and has accused President Kuchma and his allies of trying to blacken his name with corruption charges.

In October, he alleged that the authorities were plotting to kill him.