Yvette Cooper has insisted she is unaware of any attempt by Lord Mandelson to suspend the Labour leadership election by persuading the three frontbench contenders to quit en masse.
Ms Cooper said she has not had any discussions with the peer amid claims that he asked her, Andy Burnham and Liz Kendall to quit the contest in a bid to stop left winger Jeremy Corbyn from winning.
The shadow home secretary also said she was "not aware of" any approach by Lord Mandelson to her campaign team as part of efforts to persuade the three candidates to stand down and force Labour to stop the election.
The leadership race has become increasingly fractious after Mr Corbyn emerged from rank outsider to be installed as the favourite to succeed Ed Miliband.
Former prime ministers Tony Blair and Gordon Brown have both intervened to warn against voting for Mr Corbyn while others have called for the process to be halted amid fears that the hard left and Tories have attempted to infiltrate the contest.
The Telegraph has claimed that Lord Mandelson, one of the architects of New Labour, last week attempted to persuade Ms Cooper, Mr Burnham and Ms Kendall to stand down en masse in a bid to halt the race.
Ms Cooper told BBC Radio 4's Today programme: "I've not discussed this with Peter Mandelson, I gather that there was some view that the process should be stopped because so many people were joining at the last minute.
"You need to pursue this with other people if people have told you things but my focus is on the leadership election that we have."
Asked if Lord Mandelson had spoken to her office, Ms Cooper said: "Not that I am aware of. Peter is not standing in this leadership election.
"What is important is the voices of the people who are standing for the leadership election right now."
Meanwhile, deputy leadership candidate Ben Bradshaw said he was concerned that one in ten eligible voters in his Exeter constituency could be anti-Labour individuals who have registered as party supporters for £3.
He told the programme: "In my own constituency, which is probably the best organised Labour Party in the country, we have been through all of the new registered supporters and we have cross referenced them with voting records which are the best in the country.
"Consistently 10% of the new registered supporters have always said they have been strongly against Labour, they have never voted Labour, they have always voted for another party.
"That's a potential problem for the party. The party has assured all of us, the leader and the deputy leader candidates, that it has the systems in place to weed these people out, we have to take those assurances at face value.
"But you will know that three of the leadership candidates wrote last week expressing this concern, the party needs to be confident that its processes are robust."
The pair spoke ahead of a speech in which Mr Burnham is set to reach out to Mr Corbyn by offering him a role in "rebuilding the party" if he wins the election.
The shadow health secretary will warn that it will be "unforgivable" if infighting after the result is announced on September 12 prevents Labour standing up to the Tories.