The U.K. Independence Party, often seen by critics as a vehicle for ultra-right tendencies dressed up in Westminster rhetoric, promised a "political earthquake" when British voters return to polls next year to elect European Parliament members.
A seat in Westminster Parliament was also a matter of time, UKIP leader Nigel Farage told the BBC.
Thursday's by-election in Eastleigh, southern England, was triggered when former Liberal Democrat Cabinet Minister Chris Huhne was convicted of perverting the course of justice over driving license offenses.
Contrary to media predictions, center right Liberal Democrats retained the seat when Mike Thornton beat both Labor and Tory candidates but found UKIP's Diane James a close second.
Thornton's victory took the heat off junior partner and Deputy Prime Minister Nick Clegg but more striking was UKIP's gain, which strengthens British voices for downgrading EU ties.
The European Union's latest economic troubles -- soaring jobless rate, slow growth and Italian election deadlock -- were picked up by euroskeptics who want London to claw back decision-making powers ceded to Brussels and to have more say in how the union's 27 members spend taxpayers' money.
Cameron has called the European Union wasteful in its allocation of resources and criticized its 17 eurozone members for not doing enough to cut spending. Britain is outside the eurozone and keeps the pound as its national currency.
However, Cameron's frequent admonitions to Brussels are now bettered by UKIP, which has drawn disenchanted voters from a wide political spectrum -- nationalists who want to leave the European Union and oppose immigration, conservatives arrayed against same-sex marriage and opponents of education reforms.
As in mainland European countries, this angry constituency cites every negative news from the European Union to push home the point that the union isn't working.
The European Union's overall January jobless rate, a record at 11.9 percent, is actually much worse -- 26.2 percent in Spain, 17.6 percent in Portugal with new crises brewing in Italy and Cyprus. Italian youth employment tops 39 percent.
Italy's economic woes have worsened after this week's election results that gave comedian Beppe Grillo unprecedented powers over the fate of the first hung Parliament in Italy's recent history.
Like UKIP in Britain, Grillo's 5 Star Movement wants no truck with Brussels.
A total of 163 Grillo supporters won seats, 109 of the 630 seats in Italy's lower house and 54 in the 315-member Senate.
The Italian gridlock sent shockwaves through the eurozone and pitted Brussels against a hard choice of having to talk to Grillo. Analysts warn the Italian crisis may prove more hurtful to the European Union than the ongoing Greek tragedy.