Jeremy Corbyn's hopes of uniting Labour MPs were dealt a blow when a number of prominent front benchers said they would not serve in his shadow cabinet.
Mr Corbyn won the race to replace Ed Miliband as Labour leader with almost 60 per cent of more than 400,000 votes cast by party members and supporters, trouncing rivals Andy Burnham, Yvette Cooper and Liz Kendall in the first round of counting.
In an acceptance speech to cheering supporters at Westminster, he called repeatedly for unity and announced his ambition to lead a Labour “fightback”.
Mr Miliband offered the new leader his support and called on others in the party to do the same, but made clear he will remain on the backbenches. Shadow home secretary Ms Cooper said she did not expect to take a shadow cabinet post, while shadow work and pensions secretary Rachel Reeves announced she would go to the backbenches when she returns from maternity leave.
Shadow communities secretary Emma Reynolds and shadow health minister Jamie Reed also said they would not serve under Mr Corbyn.
Ms Kendall, shadow education secretary Tristram Hunt and shadow chancellor Chris Leslie had already indicated during the three-month leadership campaign that they would leave the front benches if the Islington North MP won.
However, shadow business secretary Chuka Umunna – a prominent centrist who withdrew from the contest – issued a plea for the party to "come together" behind its new leader.
In a post on Facebook, Mr Umunna said: "Now the contest is over, we must respect the result, come together and focus on providing the most credible and effective opposition to the Tories."
The scale of Mr Corbyn’s victory gives him a strong mandate and will make it difficult for any disgruntled centrists to mount a challenge before the 2020 general election.
Mr Corbyn said the campaign “showed our party and our movement, passionate, democratic, diverse, united and absolutely determined in our quest for a decent and better society that is possible for all”.
His first act as leader was to attend a “Refugees Welcome Here” rally, and he said his first day at the helm of his party in parliament would be spent opposing Government plans to “shackle” trade unions by imposing higher thresholds for strike ballots.
The Labour leader faces David Cameron across the despatch box at Prime Minister's Questions on Wednesday.
Arriving at The Sanctuary pub to celebrate with members of his team and other supporters, Mr Corbyn said he was “a bit surprised” at the scale of his majority, which he said amounted to a “fantastic mandate for change in British politics, with a fantastic enthusiasm for real democratic politics”.
Asked whether he now faced a challenge in building a shadow cabinet without several senior figures who had ruled themselves out, the new Labour leader said: “There’s going to be an inclusive, open process. I hope everyone will recognise the mandate we’ve received and that party members expect our party to deliver for them in Parliament.”
Lord Soley, a former chairman of the Parliamentary Labour Party, said he feared victory for Mr Corbyn "may have handed the next election to the Tory party". Comparing the choice of leader to that of Iain Duncan Smith taking charge of the Conservatives in 2001, Lord Soley said: "They very quickly got rid of IDS. I do not see that happening as quickly with Labour."
Labour's former London mayor Ken Livingstone played down the significance of opposition to Mr Corbyn in the parliamentary party, however, saying: "Some will object a bit but the moment Jeremy starts to do well in the polls, those doubts will go."
For the Conservatives, Defence Secretary Michael Fallon said Labour was now "a serious risk to our nation's security, our economy's security and your family's security."
He said: “Whether it’s weakening our defences, raising taxes on jobs and earnings, racking up more debt and welfare or driving up the cost of living by printing money – Jeremy Corbyn’s Labour Party will hurt working people.
“This is a very serious moment for our country - the Conservatives will continue to deliver stability, security and opportunity for working people.”
The UK’s largest trade unions, which supported Mr Corbyn, welcomed his victory. Unite general secretary Len McCluskey said voters could now look at Labour and see that it stood for “fairness, justice, peace and strong communities”.
Unison general secretary Dave Prentis said Mr Corbyn had "ignited a spark of hope, a spark that had been dampened for decades."
Mr Corbyn's landslide success is a repudiation of the centrist policy programme initiated under Tony Blair and a triumph for a grassroots campaign build around opposition to austerity, action on income inequality and a radical-left foreign policy.
Mr Corbyn, a serial party rebel who was relatively unknown outside the party before the campaign, opposes Britain’s nuclear weapons programme, says the Bank of England should print money to fund infrastructure and favours the nationalisation of energy companies and railways.
Sinn Féin president Gerry Adams welcomed Mr Corbyn's victory, describing him as a "pioneer in peacemaking".
"He talked to Sinn Féin when others wouldn't... He's a friend of Ireland. He will be an outstanding leader of his party. He obviously has a huge challenge in terms of the big defeat that they had."
