Labour chairman Hazel Blears is to launch a bid for the deputy leadership, urging the party not to distance itself too radically from Tony Blair.
Ms Blears is seen as a close ally of Tony Blair
She will be the sixth MP to enter the race to replace John Prescott when he stands down later this year with the prime minister.
Ms Blears, seen as a Blairite, is to make an announcement on Saturday.
She told the London Evening Standard it would be a "great mistake" to ditch what Mr Blair stands for.
Some other challengers - including ministers Peter Hain and Harriet Harman - have been seen to be trying to reconnect with Labour's core voters, by criticising huge City bonuses and the wealth gap.
But Ms Blears told the newspaper: "We must never forget that we need success to help deliver prosperity and support success to deliver everything else we have to do."
And the Salford MP said if Labour was seen as a party which only helped the poor and vulnerable, it risked alienating voters who had not traditionally backed them.
"People voted for Tony Blair in considerable numbers. They will take a dim view of us if we say, in effect, 'that was all rubbish and we're different now'," she said.
A source told the BBC last week that she had received a "strong" show of support from MPs and party members.
She is expected to formally launch her campaign in her Salford constituency on Saturday.
Her entry into the race means there are now six declared runners for the deputy's job, with Cabinet ministers Hilary Benn and Alan Johnson and backbencher Jon Cruddas, in addition to Ms Harman and Mr Hain, also in the race.
Ms Blears, 50, who was promoted to the Cabinet last May, has consistently defended Blairite policies from tuition fees to the Iraq war.
But her decision to join a demonstration against the closure of a maternity unit in her constituency prompted her political rivals to accuse her of "double standards", for campaigning against government policy.
And she faced criticism from potential rivals for the deputy leadership that she has been campaigning behind the scenes for some months - something she denies.
Chancellor Gordon Brown is still seen as the firm favourite to replace Mr Blair, but on Wednesday, veteran left winger Michael Meacher said he wanted to challenge him for the party leadership.
He said Labour members deserved a say in the party's future direction and insisted it was not a "foregone conclusion" that Mr Brown would be the next prime minister.
He joins fellow left-winger John McDonnell as a potential candidate.
Both men must now gain the backing of 45 Labour MPs to get on to the ballot and trigger a contest.