Australian Prime Minister Kevin Rudd has challenged the opposition to prove claims that he abused his position to help a friend's car-dealing business, BBC reported.
Mr Rudd said opposition leader Malcolm Turnbull must resign if he fails to produce an e-mail on which the claims are based by Monday afternoon.
Mr Turnbull has called for the prime minister to stand down, accusing him of lying to parliament over the issue.
Analysts say it is the biggest test of Mr Rudd has faced since he was elected.
The row has been brewing for weeks, since it emerged that car dealer John Grant lent a car to Mr Rudd for use in his constituency.
Mr Grant is a friend and neighbour of the prime minister, and opposition politicians believe Mr Rudd tried to help him secure money from a Treasury fund called OzCar to help his business.