A small anti-European Union party has rocked the British political establishment by scoring a better-than-expected result in elections for the European parliament. The election is considered another setback for British Prime Minister Tony Blair.

Euro-skeptic British voters have shocked the ruling Labor Party and the opposition Conservative Party in elections for members of the European parliament.

The results cast a shadow over Prime Minister Blair's drive to cement Britain's relations with the European Union, notably through a new EU constitution. He is due to attend an EU summit Thursday, amid opposition calls that he not sign a draft of the document in light of the election results.

Mr. Blair's Labor Party received the support of just more than one in five voters, its poorest showing in any election in more than 80 years. The Conservative Party did not fare much better, garnering just more than one in four votes, a 10 percent drop from the last EU election, in 1999.

The United Kingdom Independence Party, which would pull Britain out of the European Union, scored a surprising third-place finish, just five points behind Labor and ahead of the pro-EU Liberal Democrats.

A UKIP leader, Nigel Farage, said the major parties are out of step with the public's anti-EU sentiments.

"What we have done is we have allowed some people to re-enter the democratic process who for the last 10 years have not been voting for anybody," said Mr. Farage. "And the message out of tonight surely must be that the entire political class is headed in one direction but the vast majority of ordinary Brits went ahead in another direction."

A Labor Party leader, Health Secretary John Reid, told British radio he was disappointed with the results, but he said most of the UKIP votes came from former Conservative Party supporters.

"It was, I think, a particularly disastrous night for the Conservatives," he said, "and it has been a good night across Europe, as it happens, for parties of protest and well as parties who are, in Britain, against the European Union."

Mr. Reid says the Blair government will forge ahead with stronger ties to Europe while protecting its sovereignty on issues such as taxes and defense, what Britain calls "red lines."

"We believe that Europe brings huge and undoubted benefits to the people of this country," said Mr. Reid. "On the constitution, we have made plain that we have red lines that we will not concede. The challenge for us, and for the United Kingdom at the council that is coming up, is to get a treaty which will make the enlarged Europe work effectively when we need it, and which protects our national choices."

This was the second electoral defeat for Mr. Blair in the span of four days. The Labor party finished in third place in local council elections last Thursday, as many voters turned against the party because of Mr. Blair's decision to take Britain into the war in Iraq.

Some anti-war Labor activists wonder if Mr. Blair is now an electoral liability, but he has vowed to lead the party into the next general election, expected in about a year.