David Cameron is urging fellow European Union leaders to embrace reform, as mainstream parties fight to salvage their positions after the spectacular success across the continent of eurosceptic parties.
The Prime Minister will meet heads of government of the 28 EU states for a scheduled summit in Brussels which will be dominated by the political earthquake of Sunday night, which saw Ukip lead the polls in Britain, the far-right Front National come first in France and the extreme-left Syriza movement take top spot in Greece, while the eurosceptic Five Star movement came second in Italy and the anti-euro Alternatives won seven seats in Germany.
Ukip leader Nigel Farage, who almost doubled his tally of MEPs to 24, was setting his sights on Westminster, declaring that his "people's army" was marching on Newark in the hope of overturning a 16,000 Conservative majority in the June 4 by-election in the Nottinghamshire seat.
Mr Farage said that a strategy of targeting seats where Ukip now has local council representation should deliver the party "a good number" of MPs in next year's general election, potentially leaving it holding the balance of power and in a position to ensure a referendum on leaving Europe.
Meanwhile, Deputy Prime Minister Nick Clegg's efforts to fight off demands for his removal as Liberal Democrat leader were bolstered by the announcement of potential successor Vince Cable that "there is no leadership issue".
After the party lost all but one of its 12 MEPs and trailed fifth in the euro-elections, Lib Dem MP John Pugh called for a swift review of its leadership and strategy, making no secret of his personal preference for the Business Secretary to take over.
"If we carry on as usual, we are like the generals at the Somme, because these losses are horrendous," said the Southport MP.
And there was a further setback for the party when The Guardian reported that an ICM poll commissioned by a supporter had found that all four Lib Dem seats surveyed - including Mr Clegg's own Sheffield Hallam, Cambridge, Redcar and Wells - would be lost if he stays leader.
But Mr Clegg said he wanted to "finish the job" and insisted it had not crossed his mind to resign.
"At the point when our big decisions, our big judgments are being vindicated, we are not going to buckle, we are not going to lose our nerve, we are not going to walk away," he said.
Mr Cable, whose absence in China may have been the reason for a delay of several hours in responding to the party's disastrous results, issued a statement to warn: "Now is not the time for infighting and introspection. The party must hold its nerve."
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A Lib Dem spokesman said that the ICM poll had "clearly been commissioned and leaked for political purposes" and bore no relation to results in Sheffield Hallam on Thursday, when Lib Dems outpolled Labour by 38.7% to 23.6%.
Ed Miliband will seek to regain the initiative after a disappointing set of elections by returning to the campaign trail in the key target seat of Thurrock, Essex, with a speech designed to persuade disgruntled voters that Labour has answers for millions of people who feel "locked out of our economy or let down and ignored by politics".
Former minister Frank Field predicted a "catastrophe" for Labour if it maintained its position that a referendum should be held only if new powers are transferred to Brussels - something Mr Miliband does not expect to happen in the next Parliament.
"We have to offer a referendum. Ordinary voters think Labour's position is untenable, the Birkenhead MP told the Daily Telegraph.
But Mr Miliband insisted: "We set out our position for the general election on the European referendum. We will have a referendum if there is any transfer of powers from Britain to the European Union."
Writing in The Times, Labour's former cabinet ministers Alan Johnson and Alan Milburn urged Mr Miliband to "have the courage of his convictions and come out fighting" and not to engage in a "Dutch auction of ever tighter immigration controls".
In a series of phone calls with EU leaders over the past few days, Mr Cameron pressed the case for the reforms which he wants to negotiate before an in/out referendum in 2017.
The Prime Minister told his fellow leaders they must seize the opportunity of tomorrow's dinner to "heed the views expressed at the ballot box that the EU needs to change and to show it cannot be business as usual", said Downing Street.
A Number 10 spokesman said Mr Cameron told them that the results and low turnout in the European Parliament elections "underlined the need for reform to ensure that the EU is doing more to deliver what voters care about - jobs, growth and a better future".
In his conversations with German chancellor Angela Merkel, Lithuanian president Dalia Grybauskaite, Hungarian PM Viktor Orban, Swedish PM Fredrik Reinfeldt, Slovenian PM Alenka Bratusek and Irish Taoiseach Enda Kenny, Mr Cameron also stressed the need for consultation over the appointment of a new European Commission President after Jose Manuel Barroso's term ends in October. He was expected to make further calls before this evening's meeting.
Mr Cameron made clear that he does not accept the argument that the job of Commission President should go to Luxembourg's former prime minister Jean-Claude Juncker by virtue of his election as the candidate of the European People's Party, which emerged from the polls as the largest single party. Mr Cameron pulled the Tories out of the centre-right EPP because of its federalist sympathies.
Downing Street said that other leaders agreed that the European Council - made up of the EU's 28 national heads of government - had a "clear mandate" to nominate the next President, and that today's informal summit presented an opportunity to begin consultations on the appointment and to discuss the future direction to be taken by the Commission.
A letter to the Daily Telegraph, organised by the Business for Britain advisory board, called on political parties to spell out detailed plans to negotiate reforms of the UK's relationship with the EU.
In the letter, business leaders including Conservative donor Sir Michael Hintze and hotelier Sir Rocco Forte said the electorate "voted for change and a chance to have their say".
Politicians should now "set about explaining how they will deliver EU reform, a better deal for Britain and an in-out referendum", said the signatories.
"We urge political leaders in both Westminster and Brussels to listen and respond to the message of change that the voters have made clear they want now, not later."