The Liberal Democrats could be in Downing Street by 2009, according to one of the party's leading MPs.

Mark Oaten told a Brighton fringe meeting on Monday that the party has a "road map" to achieve the goal.

Despite cynicism in the media, Oaten insisted hat it was not a groundless claim.

The chairman of the parliamentary party argued the Liberal Democrats were hitting back at their critics in the media.

"Charles is often criticised for not being ambitious enough. You can't have it both ways," he told the meeting.

He claimed that the Conservatives are flatlining in the polls.

Based on recent trends the party was on course to overtake the Conservatives, he claimed.

"The trend of the current polls over the last six months shows we are heading in that direction," he said.

His optimism was based on past polling which puts the party at this stage in the electoral cycle at around 11 per cent.

General election publicity would give the party a "bounce" of up to six per cent which would then fuel media interest at a crucial stage in the campaign.

"My prediction is that we will go up by three per cent. All of a sudden the story is 'can you overtake the Tories?'," he said.

"By the time we get to week three we know that voters will think we can do it, and when they think that they will vote for us in larger numbers. Momentum is everything."

He also claimed comments by Conservative leader Iain Duncan Smith had revealed a man prepared to abandon voters.

"For the first time a Conservative leader has turned their back on the one nation Conservative," he said.

"What was one nation Conservatism? It was about a party that had a broad appeal. He's left the party with nowhere else to go."

Oaten claimed the Tories' support is shrinking as its membership grows old.

He added that Labour has been hit by far more than the Iraq factor.

"In the 18-34 age group we're ahead of the Conservatives. That presents us with an enormous opportunity," the Winchester MP said.

"We've got two unpopular parties. We've been winning off one unpopular party in the past."

But Oaten accepted that his party would have to define more clearly what it stood for.

"It's not a liberal thing to tax always. It's a last resort. Let's not do it for tax sake. The debate has moved on about how we deliver public services," he said.

"We also need to use the term liberal much more. We are a party that is at ease with itself and not stuffy."