The Labour MP tipped to replace Jeremy Corbyn has admitted that no seat is safe for the party after it lost Copeland to the Conservatives.
Rebecca Long-Bailey said the party can't take any of its constituencies for granted in a sign that senior Labour figures are growing increasingly concerned about poor poll results.
It came as John McDonnell, the shadow chancellor, offered to sit down with Tony Blair and have a cup of tea, admitting: "I'm willing to talk to anybody, we need advice from everybody."
The shadow chancellor has already offered to meet arch-critic Lord Peter Mandelson for a cup of tea and he has now extended that invite to Labour's former prime minister after warning that Mr Blair's Brexit speech helped deliver a defeat for the party in Copeland.
Mr McDonnell also ruled himself out of replacing Mr Corbyn as he admitted the leader will continue until 2020.
But he appeared to confirm rumours that succession planning for when Mr Corbyn steps down is ongoing, adding: "Of course we are building up a succession for the long term future and we have got some really great young talent coming through but they need more experience before eventually they will succeed."
The shadow business secretary Ms Long-Bailey is one of the young MPs who has been tipped as a possible replacement for Mr Corbyn when the time comes.
She told ITV's Robert Peston: "You can never call a seat a safe seat nowadays" when asked about the party's prospects in the Manchester Gorton by-election which was triggered by the death of sitting Labour MP Gerald Kaufman.
She added: "We have had a difficult 18 months, I don’t think anybody disputes that but if we unify now around a bold policy agenda I think we can deliver the change that Gorton and in fact the rest of the country needs."
Ms Long-Bailey also sought to play down suggestions that she could be the next party leader, branding them "fake news".
It came as Ed Balls, the former shadow chancellor, refused to rule out a return to the House of Commons after his successful stint on Strictly Come Dancing.
The former MP, who lost his seat in 2015, said: "I’m never going to say never about going back because I think politics is so uncertain.
"The issue is: can Labour have a credible, radical, patriotic alternative to the Conservatives. That’s what we need, we’re not getting enough of that despite the best efforts of Becky in that interview there.
"I’m not going to say ‘never’, but at the moment that’s not my plan and there’s a Labour leader and a Labour party in Parliament and in the country that have got to get on and deliver it.”