The anti-EU mood in Europe swept Britain on Monday as the UK Independence Party (UKIP) looked set to score a historic victory in the European parliament elections.
UKIP leader Nigel Farage hailed "the most extraordinary result in British politics for 100 years" as his party secured over 27 percent of the vote with results from 10 of the 12 regions declared.
That score put UKIP two percent ahead of the second-placed Labour Party, the country's main opposition.
The Conservatives of Prime Minister David Cameron were third with nearly 24 percent of the vote, although they were tied with Labour on 18 seats with only Scotland and Northern Ireland to declare.
UKIP had won 23 EU parliamentary seats after 10 of the regions had declared -- 10 more seats than its total at the last European election in 2009.
Farage retained his seat in south east England, and promised further gains to come.
"This is just about the most extraordinary result in British politics for over 100 years," he said.
"The penny has really dropped. We have hit very hard into the old Labour vote..., we're going to make a breakthrough in Scotland and our people's army will go from here.
"I promise you this, you haven't heard the last of us," he said.
With only a few results left to be counted, UKIP looked certain to gain its first MEP in Scotland.
But the Scottish National Party (SNP) looked set to fail in its bid to boost its share of the vote ahead of September's referendum on whether Scotland will stay part of the UK.
If UKIP were to win overall, it would be the first time in over a century that a British national election has not been won by either of the mainstream parties of the day.
The big losers were the pro-EU Liberal Democrats, coalition partners in the current national government, who managed only one EU seat, two fewer than the Green Party.
Lib Dem president Tim Farron insisted he did not regret his party's pro-Europe stance, saying "it looks like we may have paid the price but I would do it all again."
Welsh nationalists Plaid Cymru won one seat but Nick Griffin, leader of the far-right British National Party, lost his seat in north west England.
Farage said that the rise of anti-EU parties across the continent would shake up domestic politics.
"We're going to get a good number of eurosceptics elected to the European Parliament," he said. "It's going to make a very big difference in the domestic politics.
"Up until now, European integration... always seemed to be inevitable and I think that inevitability will end with this result tonight."
Cameron has promised to hold a referendum on Britain's membership of the EU if he wins next year's general election.
Farage predicted that the Labour Party could make a similar pledge in light of Sunday's results.
Foreign Secretary William Hague, of the Conservatives, said the rise of eurosceptic parties should serve as a wake-up call to European politicians.
"People use different elections to deliver different messages," Hague told BBC TV. "And certainly there is a message across Europe of disillusionment with Europe and the EU has got to hear that loud and clear."
But he sounded a note of caution about the rise of the far-right National Front (FN) in France.
When asked whether the French result was a success for a racist party, Hague responded: "In that particular case, yes. I think we should be concerned.
"That is why it is so important that the next European Commission, the European Council, the next European Parliament do get the message that there is rising discontent."
Farage ruled out any alliance with the FN, saying it was "never going to happen."
Official figures showed British turnout at 36 percent, up from 34.7 percent in 2009.