Jubilant Ukip leader Nigel Farage said his party was aiming to secure "a good number" of MPs in next year's general election after securing first place in historic polls for the European Parliament.
But Prime Minister David Cameron made clear he was not ready to countenance any pact with the eurosceptic party in the May 2015 poll, insisting that all-out victory for Conservatives was "achievable" and was the only way to guarantee an in/out referendum on Britain's membership of the European Union (EU).
Meanwhile, Ed Miliband insisted that Labour was "in a position where we can win the general election", despite disappointing results which saw the party perform strongly in London but barely scrape into second place nationally, less than a single percentage point ahead of Tories.
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After all 10 of England's regional constituencies had declared their results, Ukip was riding high with 23 MEPs and 29.07% of the vote, up 11.68 points from the last Euro elections in 2009, when it secured 13 seats.
It was the first time for more than a century that a national vote has not been won by either the Conservatives or Labour, as voters turned away from mainstream parties throughout Europe.
The worst drubbing was handed out to Liberal Democrats, who lost all but one of their 12 MEPs on a disastrous night for the party, prompting calls from senior activists for Nick Clegg to be removed as leader.
Former MP Sandra Gidley, who was among about 250 signatories to an online letter urging Mr Clegg to go, warned that the Lib Dem brand had become "toxic".
Mr Farage said Mr Clegg's position was "untenable" and said he would be "very surprised" if he remained in post until next year's poll.
But the man often tipped as most likely replacement for Mr Clegg, party president Tim Farron, was vocal in his loyalty, praising the leader for his "backbone" in standing up for the unpopular European cause while Cabinet minister Danny Alexander warned critics against plunging the party into a damaging period of introspection.
Mr Clegg's predecessor as leader Sir Menzies Campbell insisted it was not time for "a black tie... and funereal voice", insisting: "There's no doubt in my mind that Nick Clegg is the person with the courage and the resilience to take the party through to and during, and indeed after, the next general election."
With counting completed in England and Wales, Labour had 18 MEPs and a 25.35% vote share, the Tories 18 MEPs and a 24.56% vote share, the Greens three MEPs and a 7.85% share while the Lib Dems managed a solitary MEP and 6.85% of the national vote.
Mr Farage told the BBC: "My dream has become a reality. The British people have stood firm, they have backed Ukip and we have won a national election."
He dismissed suggestions that the party had peaked and would struggle to repeat its successes in next year's general election.
"Everyone keeps saying it's the high tide mark for Ukip. I think the party has got real momentum behind it," said Mr Farage.
Ukip will spend the summer developing a strategy to target efforts on Westminster seats where it has strong council representation, he said, adding: "The plan is to get a good number of Ukip MPs elected next year."
Mr Miliband insisted the local and European elections showed Labour making "progress", although there was "further to go". The party had secured gains in key areas, he said.
He said the results showed there was a "desire for Europe to work better for Britain", but rejected the idea that Labour could change its policy and guarantee an in-out referendum in the next Parliament.
"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," he said.
Mr Farage said Ukip had "dug deep" into the traditional Labour vote and Mr Miliband would become the eurosceptic's "biggest recruiting sergeant" if he continued to set his face against an in/out poll.
Mr Cameron blamed "deep disillusionment" with the EU for Ukip's stunning poll victory - but again dismissed as a "myth" the prospect of an electoral pact with the party.
The Tory leader was coming under pressure to consider a deal with Ukip, with re-elected MEP Daniel Hannan pointing out that together the parties commanded more than 50% of the national vote.
"It would be very sad if the two right-of-centre eurosceptic parties at the general election were not able to find some way, at least in marginal seats, of reaching an accommodation so that anti-referendum candidates don't get in with a minority of votes," Mr Hannan told Sky News.
But Mr Cameron told Radio 4's Today programme: "Conservative candidates will stand as Conservatives, fight as Conservatives and, I hope, win as Conservatives. We are not going to be doing deals and pacts."
The premier ducked the question of what disciplinary action would be taken against Tory candidates who struck local arrangements with Ukip, saying only: "I don't see any prospect of this happening."
Mr Cameron declined to say whether he would opt to form a coalition with Lib Dems or Ukip in the case of a hung Parliament, insisting: "Between now and the next election, I'm only going to be talking about my goal, which is all-out Conservative victory, and I believe that is achievable."
Mr Farage said he would be "relaxed" about local arrangements between Ukip and eurosceptic-sitting MPs from either Labour or the Tories, but expected them to be vetoed by Conservative high command. A national deal with Tories was not in his party's interests, he added.
Shadow business secretary Chuka Umunna acknowledged that next year's vote was not "in the bag" for Labour and said the party - which has been accused by some activists of failing to confront the Ukip threat - must now take the debate with Mr Farage's party "seriously".
"If we had a general election tomorrow, would it be in the bag? No, of course it wouldn't be in the bag," Mr Umunna told Today.
"I believe absolutely we can win the next general election, I believe absolutely that this time next year, Ed Miliband can be our prime minister, but we approach these elections with humility."
Urging Labour to engage in the debate with Ukip, Mr Umunna said: "This general election will not just be about policies, it will be about values and what you believe in as a country.
"Do we want to be a country that turns in on itself, that blames the other for all of our problems and seeks to set different groups up against each other?
"Or do we want to do what has made Britain great in the past and will make it great in the future, which is to make sure we give everyone a platform to achieve their dreams and aspirations, we hang together, we adhere to the great British values of tolerance and mutual respect for each other?
"We've got to take Ukip seriously as a party and I welcome the chance to scrutinise and debate with them their policies and their values."