Liberal Democrat leader Charles Kennedy has said he will join a demonstration against possible war with Iraq.

Mr Kennedy said he would join the Stop the War coalition march next weekend if invited - but stressed that he would support action against Iraq if it was backed by the UN.

He called on Tony Blair to make a more persuasive case for tackling Saddam Hussein, saying there was a lack of clarity in the government's approach.

If you go to war in modern times you need the broad bulk of the public behind you Kenneth Clarke

Former Cabinet minister Mo Mowlam said on Sunday she too will join the protests in London on 15 February.

Mr Blair was also warned by senior Tory Kenneth Clarke that he faced another Vietnam if the public could not be persuaded.

EU External Affairs Commissioner Chris Patten said action against Iraq risked tearing the Gulf state apart and destabilising the whole region.

Mr Kennedy, speaking on BBC One's Breakfast with Frost programme on Sunday, said he would be happy to take part in the march to set out his party's view.

He said his was a "pro-UN case", adding: "It is not anti-war come what may."

Inconsistent arguments

A spokesman for the Stop the War coalition welcomed Mr Kennedy's willingness to join the march and said an invitation had been issued to the Liberal Democrats.

Mr Kennedy said he did not doubt the prime minister's sincerity in making the case for action against Iraq.

But he went on: "But at the same time you have President Bush saying the game is essentially up, time is gone, we have made the call and that Britain will regardless go along with the American administration."

He said these were "not consistent arguments".

Kennedy said his is a "pro-UN" position

Mr Kennedy added: "Once we prosecute such a war what are we going to do about the peace? Who are we going to put in power? Who is going to police it? What is going to be the role of the British forces?"

Mr Clarke told BBC Radio Four's The World This Weekend Mr Blair would "pay" if he failed to persuade the British public that there was a proper case for war with Iraq.

"If you go to war in modern times you need the broad bulk of the public behind you - you are putting the forces at risk, you are engaging the country in military conflict in the short term, perhaps more terrorism in the long term.

"It is a broad analogy to draw between America and Vietnam but what destroyed America in Vietnam was the bulk of the American public were never really persuaded of the case for fighting in Vietnam at all."

Security council

Former Tory chairman Mr Patten told the programme that the risks extended beyond Iraq to its neighbours and beyond.

He said: "I think the real worries, in Iran and Turkey for example, are that you take Saddam Hussain off the top of the box and you find that the country blows apart, as happened in Yugoslavia.

"It may be that it proves impossible to deal with Saddam through inspection in which case the logic of the security council is that we have to move on and use other methods."

But he argued that the hawkish line being taken by many was a "pretty bleak outlook".

Financial cost

And Mr Patten said it was also important to focus on Iran and nurture its moves towards democracy and improved human rights, with the issue of weapons of mass destruction in mind.

Also speaking on Breakfast with Frost, Chancellor Gordon Brown said the financial cost of any war with Iraq could be justified.

The cost of a conflict was estimated at �3.5bn last week by a defence think-tank.

The International Institute for Strategic Studies said it could cost about �30bn a year to then rebuild Iraq, with the UK expected to pay its share.

Mr Brown said the financial impact of war could be justified by the removal of Saddam Hussein from power.