Sir Keir Starmer dismissed the list of excuses given by the leadership for the disastrous defeat and said everyone knew that Jeremy Corbyn was a key factor.
At a lunch in Westminster, the former top prosecutor insisted there was “no appetite” for a leadership challenge but repeatedly refused to rule out a future bid to be the next leader.
He told the Parliamentary Press Gallery: “I do think the Labour Party needs to be much clearer about what it needs to do to get from where it is now to getting into power 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.”
Labour lost Copeland, an area it had held since 1935, to the Conservatives in a bitter by-election sparked by the decision of centrist MP Jamie Reed to jump ship for a job at Sellafield.
Mr Corbyn suggested demographic change rather than his leadership style was to blame for the party’s poor performance.
Shadow attorney general Shami Chakrabarti, a close ally of the leader, blamed criticism of Labour from Tony Blair and Peter Mandelson, disillusionment with past Labour representatives, disunity in the party, divisions over Brexit, the media, Storm Doris and poor public transport for Labour’s defeat.
Sir Keir, who quit as part of the mass frontbench resignations in protest over Mr Corbyn’s leadership in 2016, told journalists the party “from top to bottom” needed to reflect “honestly” on the by-election results.
Labour cannot improve people’s lives without being in power and must focus on winning “at all times”, he said.
He 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.”