Jeremy Corbyn is set to visit 65 Tory marginal seats in anticipation of another general election.
The Labour leader urged his 262 MPs to think of themselves as a “government in waiting” at their first meeting since the party exceeded expectations on 8 June.
They should remain “in permanent campaign mode”, he said.
At least 22 Conservative MPs were left with majorities of less than 1,000, at the last election. Among them was Home Secretary Amber Rudd who held on to Hastings and Rye by just 346 votes.
Former Northern Ireland Secretary Theresa Villiers, was also left with a majority of 353 in Chipping Barnet.
Determined to capitalise if Britain goes to the polls again, Mr Corbyn told his MPs: “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.”
The Labour leader received a 45 second standing ovation from his team as he arrived for the Westminster meeting, with cheers audible in the corridors outside.
After winning 33 seats and achieving the biggest increase in Labour’s share of the vote since 1945, a succession of Labour politicians stood up to praise his campaign in the same committee room where he has previously been derided, booed and heckled.
It was a sharp contrast to previous meetings, when MPs urged him to stand down and accused him of wanting to make the party a “protest” movement rather than one of government.
In turn, Mr Corbyn paid tribute to party workers and supporters for the result, which saw the Conservative's lose their parliamentary majority.
They are now trying work out a deal with the Democratic Unionist Party (DUP) which would see them support their minority government.
Mr Corbyn told the MPs gathered in Committee Room 14: “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.”
He acknowledged that the party still has some way to go before it can win power, saying he wanted MPs to learn lessons about why they lost in some areas.
Labour would need a swing of just 1.6 per cent (34 seats) to become the largest party and one of 3.6 per cent to win an overall majority.
MPs who were previously sceptical about Mr Corbyn's leadership, appeared to have been won over after the meeting and several paid tribute to him after the meeting.
Among them was Chuka Umunna who described the meeting as “excellent".
He added: "Unity is the watchword. Government is the aim.”
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