Four Tory peers have been disciplined by their party for giving their backing to the UK Independence Party.
UKIP hopes to win 10 Euro seats
They were among several members of the Lords to urge voters to back the UKIP in June's European elections.
The whip has been withdrawn from Lord Willoughby de Broke, Baroness Cox of Queensbury, Lord Pearson of Rannoch and Lord Stevens of Ludgate as a result.
A fifth peer, Lord Laing of Dunphail, was not disciplined after retracting his support, a Tory spokesman said.
The peers said the open letter calling on people to vote for the UKIP was the only way to persuade party leader Michael Howard to pull Britain out of the EU.
The letter states: "We urge those who normally vote for one of the main political parties to lend their vote at the European elections on 10 June to the UK Independence Party.
"Only thus will the main parties be persuaded that their policies towards the European Union are not in the national interest.
"It is to that national interest that we all owe our allegiance."
Lord Pearson said he hoped other Tory peers would support the UKIP, saying: "We have failed to persuade Michael Howard over Europe.
Off the hook
"The only party that can save our democracy is the Conservative Party. But we hope a significant vote for UKIP can make Michael Howard see sense."
But Lord Strathclyde, who leads the Tory Party in the House of Lords, said it was "unacceptable" for Conservative members to back another party in elections.
He said Lord Laing had made clear it was a "mistake" to sign up to the letter.
Lord Strathclyde said: "A vote for any party other than the Conservatives on 10 June would be to let Tony Blair off the hook.
"Votes for minority parties can only give comfort to the prime minister who is determined to sell us out over the European Constitution."
But UKIP MEP Nigel Farage said the support of the Tory peers and seven cross-bench members showed the depth of feeling on Europe.
He said: "In the case of the five Conservatives it shows that the time has come to put country before party and is a very brave act on their part."
Foreign Secretary Jack Straw said the peers' action would bring "further chaos" to a Tory Party already "beset by panic" over its policy on the UKIP.
He said: "Today's letter is the price Michael Howard is now paying for pandering to hardline Europhobes within his party and outside who want to force Britain out of the EU."
In a statement, Lord Laing said: "On reflection, I should not have lent my name to the statement issued by UKIP earlier today."
He urged voters to "send Tony Blair a message" by voting Conservative.