Iain Duncan Smith has launched a stinging attack on the Liberal Democrats following their victory in the Brent East by-election.

Speaking to Scottish Tories in Perth on Saturday, he sought to play down the party's third place in the London constituency while arguing the Lib Dems had made a strategic mistake in revealing their true left-wing intentions.

Duncan Smith said that voters in the by-election had abandoned Labour because they "saw through" Tony Blair.

And while the election was "disastrous" for Labour, it was a "good night" for all of the prime minister's opponents.

Turning to the Liberal Democrats, he said they had made a "strategic blunder" in the by-election campaign.

"The lie that the Liberal Democrats are a moderate party of the centre was exposed like never before," Duncan Smith told delegates.

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

"Charles Kennedy, in a desperate attempt to shore up his own position, attacked Labour from the Left in Brent.

"The result is that people who voted...for Ken Livingstone in the 1980s are now voting for the Liberal Democrats."

Branding the Lib Dems a left-wing party, the Tory leader said they "pretend to be moderate when it suits them".

He said they want higher taxes and more public spending than Labour, are weak on criminals and want to scrap the pound.

It is "utter nonsense" to suggest they are the main party of opposition, he added.

Duncan Smith also turned his fire on Labour and Tony Blair.

He said the current government was the most "secretive and downright dishonest government of my lifetime".

He said the death of Dr David Kelly was one of many "shameful" episodes since 1997.

"Labour's culture of spin, of deceit, is destroying the British public's belief and faith in the democratic process," he said.

"Only with his departure from Downing Street will the culture of lies be shut down. That is why Tony Blair must go now."

Ahead of the speech, a senior Labour backbencher claimed the Conservatives planned to cut government spending by 20 per cent.

Former Scotland minister George Foulkes said it was time for the Tories to "come clean" about their true intentions.

"Following his party's abysmal showing in the Brent East by election I am sure Iain Duncan Smith will be looking forward to a break in Perth," said Foulkes.

"We know the Tories are committed to slashing spending by 20 per cent across the board.

"The reality is the Tories are now committed to cutting one in five teachers, one in five nurses and one in five police officers across Scotland.

"Mr Duncan Smith must now tell the people exactly where the axe would fall."