Labour must pull together and avoid engaging in “self-pity and recriminations” as it deals with the fallout from the disastrous election result, one of the UK’s most prominent union leaders has warned.

In her new year message Frances O’Grady, general secretary of the Trades Union Congress, stressed the importance of holding Boris Johnson’s “feet to the fire” on workers’ rights in the coming months.

“We will fight to make sure that the UK’s Brexit trade deals protect labour standards and good jobs – and stop Donald Trump getting his hands on our NHS,” she said on Monday.

Labour is launching a post-mortem into its worst election defeat of the postwar era and is preparing to formally kick off the process of electing Jeremy Corbyn’s successor next year.

On Sunday, nearly a dozen defeated Labour candidates called for a “fundamental change” in the party’s leadership and an “unflinching review” into what contributed to the largest Tory majority since Margaret Thatcher’s administration.

Big beasts lose their seats: Prominent MPs gone after election Show all 10 1 /10 Big beasts lose their seats: Prominent MPs gone after election Big beasts lose their seats: Prominent MPs gone after election Dennis Skinner - Labour Labour MP of 49 years Dennis Skinner lost his Bolsover seat to Conservative Mark Fletcher, losing 16% of the vote share PA Big beasts lose their seats: Prominent MPs gone after election Jo Swinson - Liberal Democrat Leader of the Liberal Democrats lost her Dunbartonshire East seat in a 6.8% swing to the SNP PA Big beasts lose their seats: Prominent MPs gone after election Anna Soubry - The Independent Group for Change Leader of The Independent Group for Change, formerly Conservative MP, Anna Soubry lost her Broxtowe seat, coming third behind the winning Conservatives and Labour PA Big beasts lose their seats: Prominent MPs gone after election Dominic Grieve - Independent Prominent Remain-backing MP Grieve lost the contest for Beaconsfield, coming second to the Conservatives, his former party AFP/Getty Big beasts lose their seats: Prominent MPs gone after election Luciana Berger - Liberal Democrat Luciana Berger lost the contest for Finchley and Golders Green, coming second to Conservative Mike Freer PA Big beasts lose their seats: Prominent MPs gone after election Laura Pidcock - Labour Prominent Corbyn ally Laura Pidcock lost her Durham North West seat to Conservative Richard Holden PA Big beasts lose their seats: Prominent MPs gone after election Chukka Umunna - Liberal Democrat Prominent anti-Brexit MP Chukka Umunna lost the contest for the Cities of London & Westminster, coming second to Conservative Nickie Aiken Getty Big beasts lose their seats: Prominent MPs gone after election Zac Goldsmith - Conservative Former candidate for Mayor of London Zac Goldsmith lost his Richmond Park seat to Liberal Democrat Sarah Olney PA Big beasts lose their seats: Prominent MPs gone after election Nigel Dodds - DUP Former leader of the DUP Nigel Dodds lost his Belfast North seat to Sinn Fein AFP/Getty Big beasts lose their seats: Prominent MPs gone after election Gareth Snell - Labour Labour MP Gareth Snell lost his Stoke-on-Trent central seat to the Conservatives PA

In a scathing assessment of Mr Corbyn’s leadership of the party, the former MPs and parliamentary candidates claimed “cronyism” at the top of the party and a “repeated unwillingness to stand up to the stain of antisemitism” were relayed to them during the campaign.

Andy Burnham, the mayor of Greater Manchester, also warned Labour risks becoming a “shrunken political party” in the mould of the Liberal Democrats or the Green Party as he criticised the radical manifesto “that simply wasn’t credible”.

But speaking on Monday, Ms O’Grady said: “As we face the challenges of the 2020s, all parts of the labour and trade union movement must pull together. We must be a broad church – and a bigger one too.

“My message to the labour movement for the year ahead is this: now is not the time for self-pity or recriminations. Our job is to fight for working people, not against each other.

“We need to learn the lessons of the 2019 election and listen to the working-class communities we exist to serve. We need to show humility, reflect – and then pick ourselves up and prepare for the battles ahead.”

Ms O’Grady also pledged that the trade union movement will scrutinise the prime minister on his promises to invest in schools and hospitals, get real wages rising and deliver stronger rights at work.

“We will resist his attacks on the right to strike and on the very organisations that help people get a better deal at work – unions,” she said.

Ms O’Grady continued: “Boris Johnson says he has been ‘loaned’ blue collar votes – and that he will have to earn their trust. He will be tested earlier than he thinks.

“His party forced working people to pay the price of the global financial crash. Now it’s payback time.