Ms Blears warned colleagues against 'political positioning'
Hazel Blears has broken ranks to warn cabinet colleagues to stop positioning for a Labour leadership race.
In a speech to constituents in Salford, Ms Blears told ministers to "get a grip" and stop jockeying to replace Gordon Brown when he steps down.
The communities secretary said such behaviour played into the hands of the Tory leader David Cameron.
It comes as Harriet Harman insisted there was "not an iota of truth" she was positioning for the top job.
Labour's deputy leader has been at the centre of a stream of newspaper stories in recent days, amid claims by unnamed cabinet colleagues she is preparing the ground for a leadership bid after the next election.
Some newspapers have even speculated about a "stop Harriet" candidate, with one touting Treasury Minister Yvette Cooper as a possible contender.
Ms Harman attempted to dampen down speculation in an interview with BBC Two's Newsnight.
She said: "I'm determined to support Gordon as the prime minister of this country, as he takes the country through what are very difficult economic times.
"And people in my constituency and people in the Labour Party would expect me to do that, and that is exactly what I'm doing, and nothing else."
But in a speech on Thursday, Ms Blears openly acknowledged internal strife over the leadership issue and called on colleagues to end their briefing war.
Ms Blears, who was defeated by Ms Harman in Labour's deputy leadership contest, said: "My message to my colleagues is simple: get a grip.
"Our first loyalty is to the British people. If they think we are more interested in our own jobs than theirs, they will not forgive us.
"If the mindset is all about what happens after some future election defeat, then the game's up."
She added: "All this political positioning just helps the Tories."
There was no "serious clamour" for a Conservative government, said Ms Blears, but there would be "if we fail to focus on protecting decent hard-working people," adding: "Divided parties don't win the trust of the people".
An aide insisted the speech was not aimed at Ms Harman but was "a more general point about colleagues getting a grip" following a stream of leadership challenge stories, in which a range of names have been touted.
Speculation Ms Harman was plotting a leadership bid were sparked by a passionate attack she made on City bonuses in a cabinet meeting, seen by her critics as an attempt to woo left-wingers, whose support she would need.
Some newspapers then reported cabinet colleagues' concern she was trying to "muscle in" on the G20 summit in April by staging an international women's conference at the same time.
There was also speculation that she had planted a story about Mr Brown possibly being offered the job of a global financial regulator, which was dismissed as "utter nonsense" by Downing Street.
Cabinet Office Minister Liam Byrne dismissed the stories, telling the BBC: "The media just seems to go straight to a debate that is a bit manufactured... about people positioning themselves in a contest which frankly, isn't there.
"Our focus right now as a government is on getting this country through the downturn and fighting back against the worst international recession since the Second World War."