Sir Menzies Campbell's hold on the leadership of the Liberal Democrats suffered a major blow last night after a poll showed twice as many voters thought Charles Kennedy would make a better leader.

The ICM snapshot for BBC TV's Newsnight asked more than 1000 people who would make the best leader - 49% chose Mr Kennedy while just 22% opted for Sir Menzies.

To make matters worse for the 65-year-old MP for North East Fife, the survey also said just 6% of the public believed he would make a better Prime Minister than Gordon Brown or David Cameron.

The findings will come as a major embarrassment to the LibDem leadership and overshadow the party's spring conference in Harrogate this weekend. The event is meant to be a celebration of Sir Menzies's first year in charge after Mr Kennedy resigned in January 2006 having admitted to a drink problem.

In an interview with The Herald yesterday, Sir Menzies denied he felt vulnerable or that there was any move to oust him, declaring: "There are no mumblings."

However, the poll is bound to have a destabilising effect with some party members, possibly even MPs, worried they are going backwards in the polls; some snapshots put the LibDems below 20% and even as low as 14%.

One issue that is held against the Fife MP is his age. Sir Menzies could be in his 70th year by the time of the next General Election.

However, the BBC poll showed most people were not concerned about the LibDem leader's maturity. Some 67% thought it made no difference to his ability as leader against 22% who argued it put him at a disadvantage when compared with 40-year-old Mr Cameron or 56-year-old Mr Brown.

As delegates gathered for the weekend conference, Sir Menzies sought to cast off his urbane image by insisting it was time to get "angry" on behalf of Britain's "frustrated majority".

"Last year was about stability, next year is about voicing anger on behalf of the British people," he declared.

"It is time someone spoke up for the frustrated majority. I will lead parliamentary campaigns and speak for the frustrated majority. This is what Britain needs and deserves."

He opened proceedings with a speech at a rally on the party's anti-crime strategy, which will form the centrepiece of its campaign for local and Scottish parliamentary elections in May.

Today, Sir Menzies faces a tough challenge if he is to see off a rebellion against his proposals to put off a decision on the future of Trident until 2014.

The government has already announced it will press ahead with renewing the UK's nuclear deterrent with a new generation of nuclear submarines.

While he has vowed to oppose the "premature" decision in a Commons vote later this month, Sir Menzies's official policy is to cut the missiles by half now but put off a long-term decision.

Yesterday, it emerged activists would not be faced with a straight choice between backing the official policy or an all-out unilateral decision to scrap Trident now.

Instead, opposition will be centred on a bid to take an immediate decision not to renew but to keep the deterrent in place for the rest of its lifetime - around 20 years.