The couple were convicted by a Pakistani court on corruption charges last month and received five-year jail sentences and a fine of $8.5m.

But they face further trials, including one which concerns the purchase of 8,000 tractors from Poland. According to Pakistani officials, the Polish papers contain details of illegal commissions paid by the tractor company in return for agreeing to their contract.

'Big fraud'

"The incontrovertible evidence proves that the previous government committed a big fraud on the people of Pakistan," the official APP news agency said.

The government says the money was then deposited in a Swiss bank account in the name of an off-shore company. The Swiss authorities have already provided evidence to the Pakistani prosecutors related to the tractor case.

The BBC's Islamabad Correspondent, Owen Bennett-Jones, says the Polish evidence appears to contain details of bank transfers, as well as copies of fax communications between the Polish tractor company and middle-men helping to secure the contract.

Appeal planned

Ms Bhutto's party says the PM should come clean about his own finances

Ms Bhutto and her husband have both strongly denied all corruption charges and an appeal against their conviction is expected shortly.

She says the government is pursuing her to stifle the voice of her opposition Pakistani People's Party and end her political career.

A statement from the PPP described the tractor case as "trash". It has countered by saying the Pakistani public is demanding the truth about a recent judgement in London against the family of the Prime Minister, Nawaz Sharif.