Backbench MP Diane Abbott has joined the race for the Labour leadership.
The Londoner told the BBC her bid was "serious", saying there was little between the other candidates and she would be offering Labour a choice.
The 57-year-old Cambridge graduate, who became the UK’s first black woman MP in 1987, said she was getting support from both MPs on the left and women MPs.
She is the sixth person to enter the race after David and Ed Miliband, John McDonnell, Ed Balls and Andy Burnham.
Contenders have until Thursday 27 May to gather the backing of 33 Labour MPs – a timetable some have said is too tight, particularly for less high-profile candidates.
‘More diverse’
Ms Abbott, the Hackney North and Stoke Newington MP who in recent years has been a fixture on the BBC’s This Week programme, said: "We need to speak to our supporters and speak to our members in a way that we are not speaking to them up until now."
She told BBC Radio 4’s Today programme she was "attracting support not just from the Left but from women and other MPs that want to see a more diverse range of candidates".
At the general election, Ms Abbott increased her constituency majority to 14,461, with a swing to Labour from the Liberal Democrats.
Former Health Secretary Andy Burnham, 40, announced his intention to stand for leader and "rebuild the party for new times" in an article for Thursday’s Daily Mirror newspaper.
He said the party owed a "debt of thanks" to former prime ministers Gordon Brown and Tony Blair but said he would end "stage-managed" politics.
On Wednesday Ed Balls launched his campaign, saying he wanted to "listen first, hear what the public say".
The close ally of Mr Brown said the contest was not about "Blair versus Brown" or "old Labour versus new Labour".
Left-wing MP John McDonnell said he wants to stand but the timetable makes it "almost impossible" and argues the process has been "stitched up from the start".
Ed Miliband has also said it should be longer. On his Twitter page, he wrote: "MPs/members annoyed about short nominations timetable: I have to say I agree. Need broadest possible choice & time for MPs to decide&consult".
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