Liz Kendall has rejected calls to pull out of the Labour leadership race to help block the growing challenge from hard left candidate Jeremy Corbyn.
The shadow care minister is reported to be under pressure to quit the contest to allow the moderate vote to rally behind one of her two centrist rivals – Andy Burnham and Yvette Cooper.
However an aide to Ms Kendall – seen as the Blairites' favourite – insisted that she had no intention of standing aside.
Her supporters said Mr Burnham and Ms Cooper had only themselves to blame for the growing bandwagon apparently building behind Mr Corbyn.
"It's not going to happen. This briefing is nonsense because in a preference vote it doesn't matter how many candidates there are," a spokesman said.
"This is deliberate misleading briefing that says more about Andy and Yvette's problems following their flip-flop on welfare.
"But more than that, we're in this to win. Liz is going to continue setting out how she believes Labour can win again in 2020 if we make the right choices now."
Amid bitter recriminations at the top of the party after one opinion poll put Mr Corbyn on course for a shock victory, Lord Mandelson warned that Labour's future as a viable party of government was under threat.
The former cabinet minister who was one of the key architects of New Labour said the party was struggling to deal with the "terrible legacy" left by Ed Miliband.
"Those of us who stayed and fought to save the Labour Party in the 1980s will be experiencing a growing sense of deja vu," he told The Times.
"The last five years have left us with a terrible legacy to overcome with the existence of the Labour Party as an effective electoral force now at stake."
Mr Burnham last night made a surprise declaration that he would be prepared to serve in the shadow cabinet if Mr Corbyn became leader.
Ms Cooper left the door open to remaining on the front bench but Ms Kendall ruled out taking a position with a flat "no".
During a leadership hustings on LBC, she warned that a Corbyn win would put the party out of power for "a generation".
"I think it would be disastrous for the party, it would be disastrous for the country, we'll be out of power for a generation," she said. "I don't want to be a party of protest and I wouldn't be able to stop myself from making that case."