The weekly meeting of the Parliamentary Labour Party has rarely been a happy occasion for Jeremy Corbyn. The worst gatherings were marked by fury and expletives, as MPs let rip at their leader. The best saw an eerie calm prevail. But in Committee Room 14 tonight, the mood was transformed.

After gaining 33 seats and achieving the largest improvement in Labour's vote share since 1945, Corbyn was greeted by a 45-second standing ovation. One would never have known that most of those in the room had sought his resignation just a year ago.

Corbyn told 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."

Corbyn added that the party "must remain in permanent campaign mode" and maintain the "unity and collective discipline" of recent weeks. Aided by Labour's 35,000 new members (making a total of 550,000), the leader promised to "take the fight to the Tories" in the marginal seats the party needs to win to form a government. As I noted last week, Labour needs a swing of just 1.6 per cent to become the largest party (34 seats) and one of 3.6 per cent to win a majority (64 seats).

Turning the Prime Minister's rhetoric against herself, Corbyn remarked: "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."

And Corbyn, who has often been accused by MPs of prioritising the politics of protest over that of power, declared: "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."

MPs, some of whom never expected to return to parliament, were in unremittingly high spirits. One described it as the best PLP meeting since 2010 - "because we're advancing". As he left Committee Room 14, Chuka Umunna, a long-standing Corbyn critic, proclaimed: "Excellent! Unity is the watchword. Government is the aim." Though there will be divisions to come, the days of permanent warfare are over. For the first time since Corbyn became leader, Labour presented a united front tonight.