Ed Miliband and Nick Clegg, also Britain's deputy prime minister, have resigned on Friday as leaders of the Liberal Democrats after their party was crushed at the national election.
In a speech in London, Mr Miliband said it was "time for someone else" to take over the leadership and confirmed deputy leader Harriet Harman would be interim leader.
Shadow chancellor Ed Balls was among the party's big-name casualties. It also lost its election campaign chief Douglas Alexander and its leader in Scotland Jim Murphy.
Mr Miliband was applauded by staff as he arrived at Labour headquarters in central London, reported BBC.
On Twitter, Mr Miliband said: "I am grateful to the people who worked on our campaign and for the campaign they ran. The responsibility for the result is mine alone."
Labour would "never stop fighting for the working people of this country," he added.
Earlier the Labour leader, who took charge of the party in 2010, said it had been a "very disappointing and difficult night" for his party, which is forecast to win 234 seats, compared with 258 in 2010.
Speaking after retaining his Doncaster North seat, he said his party had been overwhelmed by a "surge of nationalism."
He added: "Now I want to say to all our dedicated and decent colleagues in Scotland who have lost their seats that I am deeply sorry about what has happened."
Clegg, who in 2010 led his party to its first ever spell in government as junior partner to Prime Minister David Cameron's Conservatives, retained his seat in parliament but saw the vast majority of his colleagues lose theirs.
With nearly all the seats counted, the Liberal Democrats held just eight on Friday morning, down from the 57 they won in 2010, reported Reuters.
"Clearly the results have been immeasurably more crushing and unkind than I could ever have feared," an exhausted looking Clegg told a news conference.
"I must take responsibility, and therefore I announce that I will be resigning as leader of the Liberal Democrats. A leadership election will now take place according to the party's rules."