Party leaders are expected to set out their response to Ukip’s "political earthquake", after its victory in the European elections.
Mr Cameron has called other EU leaders ahead of a meeting in Brussels to stress the need for reform and to urge them to "heed the views expressed at the ballot box" after Ukip stormed to the top with 27.5 per cent of the vote and support surged for other eurosceptic parties.
The Ukip leader Nigel Farage, who almost doubled his tally of MEPs to 24 and has set his sights on Westminster in next year's general election, declaring that his "people's army" was marching on Newark in the hope of overturning a 16,000 Conservative majority in the 4 June by-election in the Nottinghamshire seat.
Ed Miliband will return to the campaign trail in the key target seat of Thurrock, Essex, where his speech will attempt to regain the trust of disgruntled voters in the Labour party and his leadership after a disappointing set of elections.
The Labour leader was facing pressure to switch tactics and offer an in/out referendum on Britain's EU membership in the hope of staunching the flow of votes to Ukip.
Meanwhile, Deputy Prime Minister Nick Clegg has continued to reject demands he resign as the leader of the Liberal Democrats, a position which has been bolstered by potential successor Vince Cable’s insistence that "there is no leadership issue".
His firm stance could prove ineffective however, after more than 340 members signed an e-petition calling on Mr Clegg to stand down. Crucially, they come from 150 different local parties, meaning that if half their parties are persuaded to move against the leader, there will be an election.
John Pugh, Lib Dem MP for Southport, called the Euro results “abysmal” and “worrying”, adding: “To call for business as usual is ludicrous.”
The Conservative former deputy prime minister Lord Heseltine advised Mr Cameron the dramatic surge in support for Ukip was the result of discontent over the economic downturn and would fade.
He insisted that there should be "no pact under any circumstances" with Mr Farage's party.
"It was the place to go to protest about certain things that have been happening which they associate with Europe,” he told BBC2's Newsnight, “but the real problem is the recession. Whenever you get a recession of this sort, mid-term election results find a protest point. It used to be the Lib Dems."
Labour former cabinet ministers Lord Hutton and Alan Milburn wrote to The Times to caution Mr Miliband against "Dutch auction of... ever tighter immigration controls" and instead advised him to "have the courage of his convictions and come out fighting".
Mr Farage has set Ukip’s sights on winning next week’s Newark parliamentary by-election and listed many of the seats the party will target at the general election. They include Thanet South, Folkestone and Hythe, Great Grimsby, Boston, Yarmouth, Portsmouth, Plymouth, Aylesbury, Rotherham, Eastleigh and parts of Cambridgeshire.
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