Liberal Democrat party president Tim Farron MP has used a speech at his party's conference to deliver a withering attack on the Conservative Party while seeking to boost his party's morale amid low poll ratings.

He told the audience in Birmingham on Sunday that while the Lib Dems had made the right decision to enter government with the Tories, the alliance was a temporary one.

"The picture of the coalition being a marriage is a depressing one isn’t it?" he said. "If it’s a marriage, well its a good natured one, but I’m afraid its temporary."

He added: I don’t want to upset you and its not going to happen for three or four years but I’m afraid divorce is inevitable."

Stressing his position as a LibDem working outside of the coalition government, Farron said he was able to attack Conservative policies and politicians in a way that Lib Dem ministers were not.

"There are 18 Liberal Democrats who don’t have the luxuries that I do," he said. "They can’t just sound off if they don’t like government policy or trot through the no lobby on occasions – rare occasions – to demonstrate their disagreement."

"They are our ministers, he said. "And while I’m parading my conscience around the TV studios saying the right things, they are busy in their departments doing the right things."

Being in coalition with the Tories has "tainted" the LibDem image he admitted. But he said a Conservative government without Lib Dem ministers would be "an absolute nightmare".

"We are a radical Liberal Party putting radical liberal politics into action and blocking Tory policies every day," he said. "For how many decades have we dreamed of being able to say that?"

On Sunday morning Danny Alexander, the chief secretary to the Treasury, announced plans to crack down on tax avoidance by the wealthy.

Farron said that the "contempt for society" shown by those found guilty of looting across English cities in August was matched by the super-rich who sought to avoid paying their fair share of tax.

"And lets be honest, we are sharing power with a bunch of people who think that this is OK," he said.

The party president, often talked of as a future leader of the party, also took care to praise Nick Clegg and his "tireless" leadership.

"This summer, Nick hasn’t stopped," Farron said. "His schedule racing around the country meeting members, supporters and one or two former supporters, has been staggering.

"Thick skinned, warm hearted, quick witted, occasionally paint-splattered, a Liberal to his fingertips – he leads the Lib Dems, runs the country and runs rings around the Tories."

Farron also offered an apology to party activists and councillors who lost out when the party shed more than 700 seats at the recent local elections.

"May's elections really were the bottom – at least they flipping well better had be," Farron admitted. "In much of the country we got slaughtered."

But Farron also told his party's grassroots to stop "complaining" about the difficulties that came with being in power.

He said Liberals who spent decades in opposition would "have killed" to see the day the party was in government and would have "killed us for complaining about it".

His speech came as a ComRes poll for the Sunday Mirror and the Independent on Sunday showed less than half (47 per cent) of the people that voted for the Liberal Democrats in 2010 would vote for the party today.