Peter Mandelson, right, at Monday's Lords induction ceremony
Harriet Harman has defended cabinet colleague Lord Mandelson over reports of his links with a Russian billionaire calling them "unjustified" smears.
The Commons leader rebuked Tory MP Hugo Swire after he called for a debate on press reports about the peer.
She warned MPs not to use Commons business questions to make allegations against people.
Mr Swire said MPs should be allowed to debate the issue in the Commons because of the continuing press coverage.
The Tory MP asked: "You will be aware of the continuing press reports surrounding the relationship between the current Secretary of State for Business when he was EU Trade Commissioner and Mr Oleg Deripaska, who I understand is banned from the US following an FBI inquiry into his past business activities.
"Given the very serious nature of some of these allegations, can we have a debate next week to try and ascertain as to the nature of the relationship between the Secretary of State when he was EU Trade Commissioner - both in relationship to the dispute within the Russian insurance company Ingosstrakh and the past relationship with the aluminium tariffs situation in Russia?"
Ms Harman said: "I don't think business questions should be used, and the privilege that attaches to business questions, for honourable members to make smears and allegations which are unjustified."
On Wednesday, it was confirmed that an official anti-corruption post held by former Business Secretary John Hutton would not be passed on to Lord Mandelson, who took his seat in the House of Lords on Monday after being drafted back into the cabinet earlier this month.
In a reorganisation of responsibilities carried out at the time of the reshuffle, the job of the government's anti-corruption champion has instead gone to Justice Secretary Jack Straw.
The Conservatives said it was "inexplicable" that this part of the business secretary's portfolio had not been passed on to the new holder of the post and suggested it implied a lack of trust in Lord Mandelson by Prime Minister Gordon Brown.
But Downing Street insisted the anti-corruption post was not linked to the Department for Business, Enterprise and Regulatory Reform (BERR), but was a personal appointment by the prime minister of a minister of his choice.