And when asked if the Labour leader should resign, former cabinet minister Jack Straw told Sky News: "It is for Mr Miliband to make up his mind about his future."

He is due to arrive at Labour headquarters shortly, where crowds of supporters and staff members have gathered outside.

Their difficult night began with an exit poll predicting the party's tally of seats would represent Labour's worst result since 1987. It was compounded by a dismal morning which saw shadow chancellor Ed Balls lose his Morley and Outwood seat by 422 votes.

Mr Balls said that his personal sadness was nothing compared to his "sense of sorrow I have at the result Labour has achieved across the UK."

He said the country now faced five years of doubt over the union and over Britain's continued membership of the European Union.

The Tories are now projected to win 328 seats, giving them a majority of six seats. Labour is set to win 233 seats.

Labour's John Mann tweeted: "Can't say that Labour leadership weren't warned repeatedly - those who even bothered to meet that is. Never hurts to listen."

Labour also lost what was its safest seat in Scotland, with Nicola Sturgeon's party seizing the Kirkcaldy and Cowdenbeath constituency which had been held by former prime minister Gordon Brown.

Labour's former Cabinet minister Peter Hain, who stood down at the election, said: "This is devastating for us in Scotland, where an avalanche happened and swept us aside."

Former Labour cabinet minister David Blunkett spoke to Sky News:

"It's a terrible shame for the country and for the Labour Party and I just want to make it clear that instead of this weekend us going into a kind of bunker, we have got to really examine these results very carefully indeed and we have got to learn the lessons, reaching out with what Ed Miliband himself called 18 months ago 'one nation' politics.

"We must not revert to the far left. We must not allow ourselves to turn inwards."

Nick Clegg loses Cable as Lib Dems annihilated

Senior Liberal Democrats including Danny Alexander and Vince Cable have been ousted from their seats on an election night described by party leader Nick Clegg as "cruel and punishing."

The Chief Secretary to the Treasury and the Business Secretary joined former party leader Charles Kennedy and former deputy leader Simon Hughes in losing their seats.

The Deputy Prime Minister retained his own seat but saw his colleagues toppled in a series of reverses that could see the Lib Dems reduced to single figures in the Commons.

Mr Clegg will return to Westminster from Sheffield this morning to lead discussions about the "profound implications" the devastating election result will have for his party and his leadership.

"It is now painfully clear that this has been a cruel and punishing night for the Liberal Democrats," he said.

"The election has profound implications for the country and it obviously has profound implications for the Liberal Democrats.

"I will be seeking to make further remarks about the implications of this election, both for the country and for the party I lead and for my position in the Liberal Democrats when I make remarks to my colleagues in the Liberal Democrats later this morning when I return to Westminster."

Mr Cable had held the seat of Twickenham, Teddington and the Hamptons since 1997 and was made Business Secretary after the Lib Dems became coalition partners in 2010.

He was defeated by Conservative candidate Dr Tania Mathias, who claimed 25,580 votes to Mr Cable's 23,563.

Mr Cable said the Liberal Democrats' results across the country represent a "terrible night all over for our party."

"The fact is we were hit by a very effective campaign based on people's fears of a Labour government and the SNP," he said.

"Unfortunately, it has been a terrible night all over for our party, but I'm absolutely certain we'll bounce back locally and nationally.

The Liberal Democrats have so far claimed only eight seats. An early exit poll suggested the party would win 10 seats, down from the 57 seats won in 2010.

Campaign manager Lord Ashdown has written to the party's members thanking them for "fighting the campaign of your lives."

He added: "These will be tough days ahead for our party, and they could be tough times for our country too." 

The results reflect voter disappointment with Mr Clegg's party following a series of policy reversals since the Lib Dems entered coalition with the Conservatives.

The decision to do a deal with the Tories was seen as a betrayal by many voters, who saw the Conservatives as a political opponent.

A further decision not to abstain over the issue of raising tuition fees - but instead embrace and argue in favour of the policy - caused further damage to the party.