Iain Duncan Smith says the victory will backfire on the Lib Dems

The Tory leader told the Scottish Conservative Party conference in Perth the Liberal Democrats won Brent East by fighting a left-wing campaign, thereby exposing the "lie" that it is a moderate party of the centre.

"To win the seat the Liberal Democrats used the usual tricks, faithful to the words of their secret campaign handbook: Be wicked, act shamelessly, stir endlessly," he said.

The Liberal Democrats' Sarah Teather seized one of Labour's safest seats in a key north London by-election on Thursday.

Casualties

Labour's failure to hold Brent East represented a disaster for the government and Prime Minister Tony Blair too, Mr Duncan Smith said.

The victory for the party meant the Conservatives were forced into third place, but Mr Duncan Smith said the casualties would ultimately be on the Liberal Democrats' side.

"I am determined that the campaign which won them Brent will lose them seats all over Britain at the next election," he said.

I am amused that winning a by-election could be described as a strategic blunder Time right for Lib Dem surge?

Liberal Democrats are gathering in Brighton ready for this week's conference, with leader Charles Kennedy bullish about overtaking the Conservatives at the next general election.

Speaking ahead of the conference, he responded to Mr Duncan Smith's warning, saying: "Voters don't think and don't talk and don't respond to the terms of left and right.

"They want a party that's going forward, and that is the party they saw in Brent."

The BBC's political correspondent Reeta Chakrabarti said this could be the start of a fierce battle between the two leaders.

Some of Mr Duncan Smith's critics will view his speech as a crude attempt to deflect attention from the party's own poor performance in Brent East, she said.

Brent East views In pictures

Mr Duncan Smith accused the Liberal Democrats of being a left-wing party "that pretends to be moderate when it suits them".

"The Liberal Democrats are not principled, they are not moderate, and they are not to be trusted by the British people," he said.

Mr Kennedy issued a stark warning to delegates at the Brighton conference to show responsibility when discussing matters that could damage the party's image in the light of increased media attention, including a tabled debate on scrapping the monarchy.

"The added credibility of Brent and of taking 30% in the local elections earlier this year means that we come under the microscope more and we have to be more professional and more strict in our entire approach as a party," he said.

The Tory leader also said next month's Conservative conference in Blackpool would see the first draft of the party's manifesto for the next General Election.

He said it would outline more choice in public services like health and education, but he ruled out full-blown competition between private suppliers.