Labour has "no prospect" of winning a general election if things do not improve after the party's by-election loss, Sir Keir Starmer has warned.
The shadow Brexit secretary said the loss in Copeland - the first by-election gain by a governing party in 35 years - was "really serious".
He said Labour leader Jeremy Corbyn was a key factor in the loss.
Mr Corbyn has said he took his "share of responsibility" but said he would still be in place at the next election.
He suggested demographic change rather than his leadership style was to blame for the party's poor performance.
Shadow attorney general Lady Chakrabarti, a close ally of the leader, said criticism of Labour from Tony Blair and Lord Mandelson, disillusionment with past Labour representatives, disunity in the party and divisions over Brexit were among the reasons for Labour's defeat.
Copeland, in Cumbria, had been held by the party since its creation in 1983.
Although the party lost there, it managed to hold off a challenge from UKIP in another by-election in Stoke-on-Trent Central.
At a lunch in Westminster, Sir Keir said there was "no appetite" for a leadership challenge but repeatedly refused to rule out a future bid to be the next leader.
'Very bad result'
The former top prosecutor told the Parliamentary Press Gallery that Labour needed to be "much clearer about what it needs to do" because 18 points behind in the polls and losing seats like Copeland "are not the position that any party in opposition that seriously wants to get into power can be in".
"There's a huge amount of work to be done," he said.
Sir Keir, who quit as part of the mass frontbench resignations in protest over Mr Corbyn's leadership in 2016, told journalists that Labour "from top to bottom" needed to reflect "honestly" on the by-election results.
He said the party must focus on winning "at all times".
Sir Keir said: "The loss in Copeland was really serious. I don't think some of the reasons put forward are compelling.
"I'm very straightforward that if things don't improve there is no prospect of us winning a general election.
"It is a very bad result for the Labour party and we need to be honest about that.
"A number of things came up, including the direction of travel of the Labour Party, Labour's ability to communicate and understand what people are saying to them and, of course, the leadership of the Labour Party and we all know that."