Labour sources have confirmed to ITV News that Ed Miliband is preparing his resignation speech.
The Labour leader was widely tipped to quit after his party failed to return enough MPs to form a government.
Ed Miliband has tweeted to say thank you to the Labour staff in wake of his defeat at the General Election.
He describes them as a credit to the party and to the country, and takes responsibility for Labour's loss.
I’ve just thanked Labour’s staff. They are a credit to our party, and, driven by a passion to serve, they are a credit to our country.
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.
Defeats are hard, but we’re a party that will never stop fighting for the working people of this country.
Labour MP for Rochdale Simon Danczuk said he "understands" Mr Miliband will be standing down but denied the party was in crisis.
I'm not suggesting everything is tickety-boo - quite the contrary, it's not been a good night for the Labour Party, there is no doubt about that. I don't believe it is a good night for the vast majority of people in the United Kingdom because I think they would have been better served by a Labour government. It's interesting times in which we find ourselves, politically, and what Labour now has to do is examine what went wrong with the election campaign, how we can get it right, how we can appeal to the electorate, whatever policies we need to talk more about. I understand Ed Miliband is going to announce he is standing down so yes, we will move towards a leadership election. I think that should be welcomed and it will be interesting to see who puts themselves forward.
Former Labour spin doctor Damien McBride has told ITV News the party must look for "clean skins" like Chuka Umunna and Gloria De Piero when choosing the successor to Ed Miliband.
Mr McBride explained why Mr Miliband's attempts to distance himself from the Blair and Brown era proved unsuccessful.
There were cheers from the party faithful as Ed Miliband arrived at the Labour HQ with his wife Justine. He is expected to give a speech later this morning when it is expected that Mr Miliband will stand down.
Ed Miliband may step down as leader of the Labour Party by midnight tonight, according to the bookies.
William Hill's Graham Sharpe told ITV News that the odds of Miliband resigning are 8/11 - around 24 hours after he was considered a contender to become the next Prime Minister.
Meanwhile, bets are being placed on who could replace him, with suggestions including his brother and former foreign secretary David Miliband.
Andy Burnham is the favourite to become the party's new leader, with odds places at 5/2, followed by Yvette Cooper (3/1), Chuka Umunna (6/1), Dan Jarvis (7/1), Tristram Hunt (10/1) and David Miliband (11/1).
Labour leader Ed Miliband is currently the most talked about party leader on Twitter, the social media site has told ITV News.
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As he mounts his final push for Election victory, Ed Miliband has urged to people to vote Labour tomorrow so that his party can put "hard working families first" ahead of the "privileged few."
Mr Miliband said:
"I’m asking people to vote Labour tomorrow to put their family first so that we can be a country that rewards hard work again, gives opportunity to our young people and rescues our NHS.
"If I’m Prime Minister I will put working families first, not the privileged few – and that’s the choice at this Election."
Ed Miliband carving his manifesto into stone does not mean a Labour government would keep to the promises, the party's campaign boss has suggested.
Lucy Powell said Mr Miliband stood by his pledges but added: "I don't think anyone is suggesting that the fact that he's carved them into stone means, you know, that he will absolutely, you know, not going to break them or anything like that."
Her political rivals were quick to seize on the comments with the Conservative Party chairman Grant Shapps saying: "£30,000 worth of limestone doesn't change the fact that Ed Miliband will say one thing and do another."
Ms Powell later tweeted that she had been misquoted during her appearance on BBC Radio 5 Live.
Honestly Tories and others desperately mis-quoting what I said. Anyone who heard the whole interview knows I said the opposite.
The 8ft high stone was widely mocked after its unveiling at the weekend, with David Cameron labelling it a "tombstone".
Ed Miliband has told voters there are "less than 48 hours to get rid of this Tory government and David Cameron" as he placed the NHS at the centre of his campaign again.
Mr Miliband told an audience in Bedford that the health service was facing a "financial timebomb" with the Tories planning "savage cuts".
He said he wanted to put the "right values" including "care, compassion and corporation" back at the centre of the NHS.
The Labour leader said he would keep going "right till the last minutes" to rescue the National Health Service and prevent another five years of Conservative government.
ITV News' Carl Dinnen, who is following Labour's campaign, said Mr Miliband had admitted "this race could be one of the closest in our history" as he outlined the choice facing voters on Thursday.