Supporters must back Jeremy Corbyn, says Tom Watson

His comments came as shadow attorney general Shami Chakrabarti blamed factors such as the media and division in the party for two poor performances in by-elections this week.

However, Labour MP Lisa Nandy, touted as a possible party leader, said it was "a severely inadequate response" to lay the blame beyond the party for its current problems.

Speaking on ITV's Peston on Sunday, Mr Watson said: "If I've got some frustrations, it's that those people that are Jeremy's cheerleaders, that made sure that he was elected a second time last September, they should be sticking with their leader in the bad times, not just the good.

"Dave Prentis has spoken out, but I'd say to you this morning where's Len McCluskey defending his leader in this difficult time?

"It shouldn't be just down to me."

Mr Watson said it was not "a suicide approach" to stick with Mr Corbyn, as he urged the party to unite and ruled out a fresh leadership contest.

He added: "We can win an election with Jeremy Corbyn, but things have to change."

Appearing on The Andrew Marr Show on BBC One, Labour peer Baroness Chakrabarti blamed a whole host of factors for Labour's by-election defeat in Copeland, as she refrained from criticism of Mr Corbyn.

The shadow attorney general 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.

"It's not about the terrible regime, it's about the fact representatives cannot weigh people's votes anymore," she said.

"What I would say to Dave Prentis and some of these other great men of the left is it's time to unite, because constant attacks on the leadership, constant leadership elections, constant divisions, don't actually attack the issues that would allow us to present an alternative vision."

Gerard Coyne, who is challenging Len McCluskey for leadership of the Unite union, also criticised Mr McCluskey's failure to speak out.

"Len McCluskey has spent his time as general secretary of Unite pulling the strings of the Labour Leadership," said Mr Coyne.

"But as the party's facing its biggest crisis in over a generation, with the Labour leadership completely disconnected from its traditional support, Len is suddenly nowhere to be seen or heard.

"The silence of Len McCluskey is truly damning. He has driven Labour to the edge of the cliff and then disappeared in a puff of smoke as it tumbles over the edge.

"I find it utterly unbelievable that when he has been previously so obsessed with the Westminster bubble that he has conveniently lost his voice."

Shadow chancellor John McDonnell insisted Labour's problems were "not a leadership issue" but said he and other members of Mr Corbyn's top team now need to reach out and listen to different wings of the party in policy and communications development.

He told BBC 5 Live's Pienaar's Politics: "Copeland, yeah, I understand Copeland, it's hit me, very personally, very hard, it's hit personally Jeremy very hard.

"But let's turn it into an opportunity now, let's use that now to start coming together for that discussion.

"And I've got to learn some lessons about that as well, that's why I want to start listening to people like Progress and others from different wings of the party much more effectively than I have done."

Wigan MP Ms Nandy, the former shadow energy secretary, said this week's elections were "very bad results" for Labour.

"I think the trouble with looking at every factor apart from Labour is that it's just a severely inadequate response," she told Sky's Sophy Ridge on Sunday.

"If we really want to address what has been happening to the Labour party for a very long time, then we as a party need to get out of our comfort zone and start confronting some of the very difficult issues that we face."

Ms Nandy added that there was "no appetite" for a fresh leadership election, as she ruled out any plans to run for leader herself.

"Labour is in real trouble and there is no point pretending that isn't the case," she said.

Later on Sunday, Mr Corbyn will insist now is not the time to "retreat, run away or give up", despite the party suffering a humiliating by-election defeat.

The under-fire leader will say he "cannot lie and say the result in Copeland was what we wanted", but will also insist the "time has come" for his left-wing policies.

The loss of Copeland shows the "scale of how hard our task is to persuade people of our message", Mr Corbyn will say, as he addresses activists at the Scottish Labour Party conference in Perth.

Labour former deputy leader Harriet Harman said there are "painful echoes" of the 1980s in Labour's situation now.

In an interview for The Westminster Hour on BBC Radio 4, she added: "The problem is, if you face inwards, either congratulating yourself on how many members you've got or thinking that you've got a huge mandate after your election - a mandate from the members - well, a mandate from the members is important, but the real job of a Leader of the Opposition is to get a mandate from the people in a general election, to be Labour prime minister."

Fellow former Labour deputy leader John Prescott, meanwhile, said Labour must unite if it wants to avoid further defeats at the ballot box.

Writing for the Sunday Mirror, he said: "It's vital Labour's MPs and Corbyn find a better way to work and campaign together to convince the voters that we can be the next Government."

Shadow Scottish secretary Dave Anderson used his address at the Perth conference to call for party unity.

He spoke shortly after Mr Corbyn arrived at the Perth venue to chat to delegates ahead of his speech later on Sunday.

The Labour leader refused to respond to questions from reporters on the tenability of his position.

Mr Anderson told the conference: "We know we are in the fight of our lives. You know that more than most. We have had setbacks and we must get to grips with the position we are in.

"And to do that we need unity. And that starts at the top of this party with my colleagues and my comrades in the Parliamentary Labour Party.

"We won't win anything if we aren't united. And we won't win anything if we don't make a genuine offer to our people - especially those who feel that, even in Government, Labour wasn't listening to their needs and worries."