Shadow business secretary Chuka Umunna has called for Labour to unite behind the new party leader in an apparent offer of reconciliation from the party's modernising wing to leadership favourite Jeremy Corbyn.
He also welcomed a surge in party membership and support in the run up to the leadership contest saying that it was important not to "dismiss out of hand" criticism of New Labour.
Yvette Cooper and Liz Kendall have denied reports that Lord Mandelson tried to persuade the three mainstream Labour leadership candidates to quit the leadership contest and force the party to stop the election.
The move by the former politician and 1997 election campaign leader highlights maneuvers within the party to stop Mr Corbyn winning the election.
Ms Cooper said she had not been approached directly by the Labour peer, she was unaware if her campaign team had been contacted.
Ms Kendall insisted that neither she nor her office had spoken to him.
Labour's Chuka Umunna urged his party to unite around its new leader in an apparent offer of reconciliation to left-winger Jeremy Corbyn.
Some 3,000 infiltrators have been discovered and barred from voting in the Labour leadership contest, a party source has told ITV News.
The Labour leadership candidates will meet with acting leader Harriet Harman after accusations the vote was infiltrated by opponents.