London: Harriet Harman, the former Labour Party deputy leader, has said Jeremy Corbyn should take the credit for the party’s success at the general election and that she would now expect all Labour MPs to want to serve in his shadow cabinet.

Speaking before a meeting of the parliamentary Labour Party (PLP), Harman, who has been in the Commons since 1982, conceded on Tuesday that her earlier doubts about Corbyn’s leadership had been misplaced. As another influential Labour backbencher, Clive Efford, argued that the existing shadow cabinet deserved to keep their positions, Harman said she would expect former critics in the party to now be happy to serve under Corbyn. Harman told BBC Radio 4’s Today programme she had been wrong in her assumption that Corbyn could not take the party forward.

“He’s confounded those expectations. Just as, if we’d have lost seats under his leadership, he would have had to take responsibility, we’ve gained seats under his leadership, and he can take the credit for that.”

Harman said the atmosphere among Labour MPs even shortly before the election had been morbid: “We were expecting the Tories to lay waste to us. Instead it turned around, and we come back coherent, united. “The atmosphere is verging from on one hand relief to jubilant, and the Tories are in disarray. And Jeremy Corbyn has to take the credit for that, because he was the leader and he’s gone forward.”

Asked if she would serve in the shadow cabinet if asked, Harman said she had “done a very long stint” on the front bench and would not do so again.

“For other people, I think he can choose who he wants to have in his shadow cabinet. And I would expect anybody he asks in the PLP would be prepared to serve now because what he’s shown is that he can lead us forward at a time when the Conservatives were expecting to make gains.” Efford, the chair of the Tribune Group, which numbered more than 75 MPs before the election, praised Corbyn’s current shadow cabinet for sticking by the Labour leader throughout the election campaign.

There had been mounting speculation that the Labour leader, buoyed by 30 Labour gains and Theresa May’s struggles to form a government, could strengthen his team with experienced former ministers. Several Labour MPs, including Yvette Cooper, Angela Eagle and Chuka Umunna, have said they would serve in the shadow cabinet despite past criticism of Corbyn’s leadership.

Efford told the Guardian on Monday: “Jeremy has got a shadow cabinet that remained loyal and allowed him to perform extremely well during the general election. He can’t sack those people. They deserve to be rewarded for what they have done. We need to get behind those people and give them all the support [we] can.”

On Tuesday morning the shadow cabinet is due to hold its first meeting since the election. Some shadow ministers, including the shadow home secretary, Diane Abbott, were criticised for their performances during the campaign.

Tribune relaunched in April with a new mission statement that was seen as a programme to win back traditional supporters while appealing to the centre ground. Those who signed up to the document included Cooper and the failed challenger to Corbyn in last year’s leadership contest, Owen Smith.

Efford said backbench MPs who once questioned whether Corbyn could gain support among traditional voters had been won over by his performance. “We questioned whether voters would be prepared to get behind Jeremy at a general election. The opinion polls suggested we were right about that. But it has to be said that Jeremy is a brilliant campaigner and did extraordinarily well. People have had a good look at him and found that they can get behind him. They see him as a credible leader.”

Efford said he would support any moves by Corbyn to reach out to new members if roles in his team became free. “I would like to see some new faces.” On Sunday, Corbyn said he was prepared to make overtures to MPs who had been critical of his leadership, hinting that he could broaden his shadow cabinet. But the shadow chancellor, John McDonnell, appeared more resistant to changing personnel and inviting Corbyn’s former critics back into the fold. Shadow cabinet members are expected to demand clarification on Tuesday over Labour’s Brexit policy, particularly about whether the party would negotiate to remain in the single market. Apparently conflicting statements on membership have come from the shadow Brexit secretary, Sir Keir Starmer, and Corbyn.