Keir Starmer has said Labour's by-election victory shows the party is "on track" to power and said the Tories were "absolutely imploding".
The beaming Labour leader said Wakefield would turn out to be the "birthplace" of the party's next Government as he celebrated winning back a key brick in the Red Wall.
Boris Johnson had a torrid night at the polls as the Tories lost two critical by-elections, in a humiliating blow to his leadership.
The under-fire PM was more than 4,000 miles away in Rwanda when the news broke, threatening to derail his diplomatic efforts at the Commonwealth Heads of Government Meeting (CHOGM).
In a night of drama, Labour's Simon Lightwood regained Wakefield with a majority of 4,925, on a swing of 12.7% from the Tories.
The Liberal Democrats overturned a 24,000 Conservative majority in Tiverton and Honiton, in a stunning victory for the party in the Tory 'Blue Wall'.
Tory chairman Oliver Dowden dramatically resigned in the early hours of the morning, catching Mr Johnson by surprise as he took an early morning swim.
Speaking to broadcasters in Kigali, Mr Johnson admitted the results were "tough", but he insisted he wouldn't quit and vowed to "listen" to voters.
But Mr Starmer said "if the Tories had any decency they would get out the way", as he celebrated Labour's by-election win in Wakefield.
On a visit to Ossett Market, the Labour leader said: "What a judgment this is on the Tories and Boris Johnson - out of touch, out of ideas, and if they had any decency they would get out the way for the sake of the country.
"When we do form that next Labour government, and we're going to do it, Wakefield will go down as the birthplace of that."
Winning back Wakefield is a major boost for Mr Starmer, who has been facing muttered questions about his leadership in recent weeks.
The seat had been Labour since the 1930s but was seized by the Tories in 2019 when Mr Johnson snatched swathes of the so-called Red Wall.
A by-election was triggered after Tory Imran Ahmad Khan resigned earlier this year following his conviction for sexually assaulting a teenage boy.
Mr Starmer said the win was a "huge result" for the party and was "absolutely" a brick back in Labour's red wall.
"This puts us now absolutely on track for a Labour government, which is absolutely coming," he said.
Pointing to a "significant" swing towards Labour, he added: "That tells you that the next government is going to be a Labour government and the sooner the better, because the country voted yesterday in both by-elections, no confidence in this out-of-touch, out-of-ideas Government.
"This is a historic by-election as far as we're concerned."
Mr Starmer also said the Conservatives were "absolutely imploding" following the resignation of Mr Dowden.
The Lib Dems were also celebrating after a decisive win in Tiverton and Honiton, where candidate Richard Foord overturned a Tory majority of 24,000.
Mr Foord won 22,537 votes compared with Tory candidate Helen Hurford's 16,393 votes - a 6,144 majority.
The vote, which was held after Conservative MP Neil Parish resigned for watching porn in the Commons, is the second massive victory for the Lib Dems in Tory heartlands after their coup in North Shropshire in December.
Lib Dem leader Sir Ed Davey told LBC: "We are smiling here and the message from Tiverton and Honiton, the people here in Devon, is that Boris Johnson must go.
"I think they've spoken for the whole of the British people and it really is time he left."
He added that the country was "in chaos" under Mr Johnson's leadership and the Conservatives appeared to have no plan.
The Liberal Democrats had fronted a "positive" campaign in the constituency with policies that had "energised" people while the current Government falters, he said.
"I think it speaks on behalf of people - Boris Johnson really must be pushed out," Sir Ed said.
The rural Devon constituency sits in the so-called Blue Wall of Tory heartlands in the south of England - and the loss will ring major alarm bells for Conservative MPs.
Tory grandee Sir Geoffrey Clifton-Brown said there is "no doubt" it would be "difficult to hold" his seat if there were a by-election in his constituency now.
The Cotswolds MP, who is Treasurer of the Tory backbench 1922 committee, said: "I think, factually, if I were to run under a bus today it would be difficult to hold my seat. There's no doubt about that.
"I feel very sorry for all our volunteers, and indeed my colleagues, and indeed myself, who work very hard in these by-elections, but were simply defeated by the situation that we find ourselves in at the moment."
