Theresa May's premiership has no mandate and no legitimacy, Jeremy Corbyn told Labour's MPs as he declared: "We are now a government in waiting".
The Leader of the Opposition was greeted with cheers and a 45-second ovation as he arrived at the first meeting of the Parliamentary Labour Party since the election which saw Mrs May's Tories lose their Commons majority.
The scenes were in marked contrast to the difficult meetings Mr Corbyn had with his critics in the parliamentary party before the election.
He said Labour would remain on a general election footing due to the instability of the minority Tory government.
Mr Corbyn told the MPs and peers: "Last Thursday, we turned the tables on Theresa May's gamble and gained seats in every region and nation of Britain and I'm particularly delighted that we have increased our representation in Scotland.
"We increased the Labour vote by the largest margin in any election since 1945 and gained seats as a party for the first time since 1997.
"So now the election is over, the next phase of our campaign to win power for the majority has already begun.
"We must remain in permanent campaign mode on a general election footing.
"We achieved what we did last Thursday because we were a united party during the campaign and we need to maintain that unity and collective discipline in the weeks and months ahead.
"We will continue to take the fight to the Tories and I will be out campaigning around the country in Conservative marginals in those extra seats we need to gain to deliver the government for the many that almost 13 million people voted for last week.
"Now as Parliament returns, we have a Government in complete disarray still unable to reach an agreement, it seems, with the DUP and desperately delaying the Queen's Speech and Brexit negotiations.
"Far from being strong and stable, the Government Theresa May is putting together is weak, wobbly and out of control. This is a Government on notice from the voters.
"Theresa May has no mandate and no legitimacy for policies that do not have the support of the majority of the British people.
"We are now a government in waiting and we must think and act at all times with that in mind.
"That is our responsibility to the huge numbers who voted for our manifesto last week: a programme to transform Britain for the many that caught the imagination of millions.
"This was a remarkable result achieved because we stayed united and worked as a team and I have no doubt together we can win the next general election, whenever that may be."