Labour must mount a new offensive on the Liberal Democrats to ensure the Tories do not win by the "back door", Cabinet minister Peter Hain has said.
Hain started his political life as a Liberal
The Lib Dems have scored recent by-election successes against Labour.
Mr Hain told The Independent newspaper Labour supporters needed a "serious wake up call" and said they must put Lib Dem policies under closer scrutiny.
The Lib Dems say their support goes deeper than their stance on the Iraq war and Mr Hain is recognising reality.
Leader of the House of Commons Mr Hain was himself a Liberal in the 1960s.
His comments come after reports that Lib Dem leader Charles Kennedy wants a strategy review so his party targets Labour as well as the Conservatives at the next election.
The Lib Dems have won Brent East and Leicester South from Labour in by-elections and narrowly missed overturning a huge Labour majority in Birmingham Hodge Hill.
With a general election expected next spring, Mr Hain says the Lib Dems cannot directly win Tory-Labour marginal seats.
"But Labour supporters, by plumping for [the Lib Dems] or staying at home, could inadvertently wake up with a Tory MP and therefore potentially let Michael Howard in through the back door," he told the newspaper.
"There has got to be a really serious wake-up call to progressive voters and natural Labour supporters.
"If they don't want a Tory government, there is really only one way to achieve that in Labour-held seats - and that is to vote Labour.
"It's important that the searchlight is now put on Lib Dem policies, staring at their conference. They have got away with blue murder for decades."
'Not one trick show'
Mr Hain said Lib Dem proposals to scrap the New Deal would cut efforts to help disabled people into work. He also attacked plans to abolish child trust funds.
But a Lib Dem spokesman said it was better to spend the child trust fund money on improving early years schooling rather than a one-off cash handout.
The New Deal plans were part of a drive to cut central targets and get decisions on such programmes taken at a regional level.
The spokesman added: "What Peter Hain is saying makes sense. We have picked up Labour voters in Labour heartlands.
"The media have been slightly lazy in the sense that they have put it down to Iraq. But it's not all down to Iraq."
Lib Dem proposals for replacing council tax with a local income tax and abolishing student tuition fees have defined the party in the minds of voters, he argued.