The “first task” of the next Labour leader will be to explain why the party has not won an election for a decade, according to its former deputy leader.
Tom Watson said shadow cabinet members wanting to succeed Jeremy Corbyn will face “particular pressure” over the party’s last manifesto.
Mr Corbyn confirmed in December he would stand down, after Labour suffered its worst election defeat since 1935.
Jess Phillips is expected to join the race to replace him later.
The Birmingham Yardley MP would follow shadow foreign secretary Emily Thornberry and shadow Treasury minister Clive Lewis, who have already confirmed their intention to run.
Other MPs are expected to put their names forward, with shadow Brexit secretary Sir Keir Starmer, shadow business secretary Rebecca Long-Bailey and Wigan MP Lisa Nandy all saying they are seriously considering running.
There will also be an election for a new deputy leader, with shadow justice secretary Richard Burgon announcing his candidacy on Twitter, and shadow education secretary Angela Rayner receiving the backing of Ms Long-Bailey.
Under current rules, would-be candidates for both the leader and deputy leader roles must first be nominated by more than 20 MPs.
They must also secure nominations from at least 5% of Labour’s constituency parties or three affiliated bodies – two of which must be trade unions.
A timetable for the leadership election – and any rule changes – are set to be decided by the party’s National Executive Committee (NEC) on Monday.
‘Responsibility for failure’
Speaking on BBC Breakfast, Mr Watson said it was “too early in the race” to give his backing to any candidate.
He told the programme: “For whoever wants to lead the Labour Party, their first task is to explain to 500,000 members of the party why Labour lost and why Labour hasn’t won for a decade.”
Mr Watson said he “didn’t sign up” to the party’s 2019 manifesto, but he accepted he “must take responsibility for the failure as well”.
“So,” he added, “those members of the shadow cabinet that are running for leadership and deputy leadership of the party… have got a particular pressure on them, as they do have to explain whether they think that particular manifesto was the right one or not.”
Ms Phillips, who hinted on Twitter she would be entering the race, has been an outspoken critic of Mr Corbyn’s leadership and has not held a shadow cabinet post under him.
Former Labour MP Melanie Onn – who lost her seat in Grimsby at the last election – said she would support Ms Phillips for the top job if she declared later.
She told the Today programme she was looking for a leader “who will transcend normal politics”, who would make “strong committed arguments”, and had a “strong personality” to stand up to Boris Johnson.
She said Ms Phillips, who visited her former constituency during the election campaign, was in a “very good position to reach out” to Labour voters who had moved away from the party.