Labour's leaders must take "a long hard look" at what is not working following the party's defeat in the Copeland by-election, deputy leader Tom Watson has said.
Although he described the result as "hugely disappointing", Mr Watson told Scottish Labour's conference in Perth that it was not the time to replace Jeremy Corbyn.
Mr Corbyn has made it clear he is determined to continue as leader, despite warnings that the party was on course for a "catastrophic" general election defeat after the Conservatives snatched the Cumbrian seat, which had been held by Labour since 1935.
Mr Watson told delegates he would not "sugar coat" the result.
After welcoming a win in the Stoke-on-Trent Central seat, Mr Watson said: "Gareth Snell will be a great MP, and sending Paul Nuttall back home with his tail between his legs is exactly what he deserved.
"But I'm hugely disappointed that Gillian Troughton won't be joining him in Parliament as the MP for Copeland.
"That means that all of us with leadership roles in the Labour Party need to have a long, hard look at ourselves and what's not working.
"Seven years into a Tory government, we shouldn't be facing questions about whether we can retain the seats we already hold.
"Our job at the next election is to gain over 100 seats. Keeping what we have is supposed to be the easy bit.
Here in Scotland, you've seen what happens when Labour's long-term supporters stop voting Labour.
"We can't afford to have that happen in England too.
"I've said it a lot recently. This is not the time for a leadership election. That issue was settled last year.
"But we have to do better. We cannot sustain this level of distance from the electorate, from our natural supporters."
Mr Corbyn said the result was "very disappointing" but made clear he would resist calls to resign.
"I was elected to lead this party. I am proud to lead this party," he told reporters.
"We will continue our campaigning work on the NHS, on social care, on housing."
One leading trade union figure warned Mr Corbyn he had to "turn things around" and show he could deliver a Labour government, while another said he needs to take responsibility for improving the party's fortunes "sooner rather than later".
Following a tumultuous election night on Thursday, a jubilant Theresa May travelled north to Copeland to hail an "astounding" victory for Tory candidate Trudy Harrison.
Her triumph was the first gain for a governing party in a parliamentary by-election since 1982.
Later, Mrs May told the Conservative Councillors' Association conference in Lincolnshire that Labour's "devastating" defeat showed it is "out of touch with the concerns of ordinary working people" and that only the Tories "can truly call itself the party of working people".
John Woodcock, MP for neighbouring Barrow-in-Furness, warned the party was heading for a "historic and catastrophic defeat" at the general election.
Senior Labour backbencher David Winnick said Mr Corbyn was an "obstacle" to victory and should consider his position as he is "simply not acceptable" to traditional Labour voters.
Labour's shadow leader of the Lords, Baroness Smith, described the Copeland defeat as "devastating" and said Mr Corbyn should "think long and hard" about his response.
Dave Prentis, general secretary of the Unison trade union, said while the blame for a "disastrous" defeat did not lie solely with Mr Corbyn, he had to take responsibility for what happened next.
"Nurses, teaching assistants, care workers and ordinary people everywhere need a Labour government. Jeremy has to show he understands how to turn things around and deliver just that," he said