The UK Independence Party hailed a "sea change" in British politics today as it emerged as a serious nationwide threat to Britain's three main parties after making unexpectedly big gains in the local elections.

A BBC projected share of the national vote put Labour ahead on 29 per cent; the Conservatives on 25 per cent; Ukip on 23 per cent; the Liberal Democrats on 14 per cent and others on 9 per cent.

It showed the Conservatives winning 36 per cent (down nine points on 2009 when the same seats were last fought); Ukip on 24 per cent (up 20 points); Labour on 16 per cent (up six points), the Liberal Democrats on 15 per cent (down 11 points); the Green Party on 3 per cent (down one point) and others on 6 per cent (down five points).

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Ukip, which supports withdrawal from the European Union, is on course to exceed experts' predictions and have more than 100 councillors across the country.

Ukip showed it could perform well in the North by coming a strong second in the South Shields parliamentary by-election, caused by the resignation of David Miliband. Labour retained its safe seat with a reduced majority of 6,505. Ukip scooped up votes from the Conservatives and Liberal Democrats to land 24 per cent of those cast. The Lib Dems came a humiliating seventh.

Nigel Farage, the Ukip leader, said: “This is a real sea-change in British politics." He said: “The people who vote for us are rejecting the establishment and quite right too. I don't think these votes are going away quickly. After the Conservative Cabinet minister Kenneth Clarke dismissed Ukip as “clowns,” a triumphant Mr Farage quipped as he toured the TV studios: “Send in the clowns!”

In the council elections, the Conservatives appeared to be the main victims as three out of four Ukip gains came at their expense. The Tories lost control of Dorset and Gloucestershire, where Labour and Ukip made gains, but retained Essex, Somerset, Hampshire and Dorset.

The Lib Dems are braced for a disappointing set of results and coming fourth in the share of the vote. But they insisted their vote was holding up in the areas where they hold parliamentary seats, giving them hope of retaining them at the 2015 election.

Labour increased its share of the vote by six or seven points and gained control in Derbyshire. Officials claimed it was “doing better than expected” in the South of England.

Although Ukip will not run any authorities, its strong showing across the country will shake the other parties and suggests that Mr Farage's party could come top in next year's European Parliament elections.

John Curtice, professor of politics at Strathclyde University, described Ukip’s performance as “remarkable,” saying it now represented the biggest threat to Britain's three-party system since the Second World War.

John Baron, a Eurosceptic Conservative MP, described Ukip's advance as a "wake-up call" to his party. He urged David Cameron to bring in a Bill in the next parliamentary session, starting next Wednesday, to guarantee an in/out referendum on Europe after the 2015 general election.

Grant Shapps, the Tory chairman, insisted it had been a bad night for all three main parties. His message to voters was: “We get it. We have heard you.”

Michael Fabricant, a Tory vice-chairman, said: "I hope there will be some serious research about exactly what message Ukip voters are giving: none-of-the-above or specific issues."

He said "life cannot go on as normal" for the main three parties "unless they have no ambition for 2015".