Nick Clegg has vowed to play hardball with David Cameron over wealth taxes and Trident as he admitted that the Liberal Democrats had made mistakes during their first two years in coalition.

Kicking off the party's autumn conference, the Deputy Prime Minister conceded that errors had been made which would leave "scars" for a long time to come.

But he urged activists not to give in to squabbling and retreat to their "comfort zone", insisting that Lib Dem ministers were helping to making Britain a fairer place.

The rallying call came as Mr Clegg struggled to quell rumblings about his leadership, fuelled by consistently dire opinion poll ratings.

He told the gathering in Brighton there had been "some real highs and more than a few lows" since May 2010, listing crowd-pleasing achievements such as raising the tax threshold towards £10,000, increasing the state pension, and introducing the pupil premium.

The Lib Dem leader also highlighted areas where the party had blocked Conservative policies, including stopping the Trident nuclear deterrent replacement being approved in this parliament.

He announced that Treasury Chief Secretary Danny Alexander would now take charge of a Whitehall review of alternatives to the weaponry. Concerns had been raised that Cabinet Office and education minister David Laws - previously slated for the task - would not be able to give it enough attention.

"Danny has spoken out repeatedly about how expensive and unnecessary a like-for-like replacement would be," Mr Clegg said. "And there is no one better when it comes to getting value for money for the taxpayer. I am more determined than ever to find the right alternative to such a monumentally expensive replacement for a Cold War deterrent."

Mr Clegg stressed it was only "half-time" in the coalition's term of office, and rubbished suggestions he would step down before 2015 by promising to fight for the party's values right up to the general election.

"One of the most important ways we can do that is by making taxes fairer," he said. "Lower taxes on work and more on unearned wealth. I want to reward people who put in a proper shift, not those who sit on a fortune. People for whom a bonus means a few extra quid at Christmas, not a million pound windfall."