Facing political fallout from the Deputy Prime Minister's gaffe, a Downing Street spokesperson said Clegg's view of the conflict was already well-known.

"The coalition government has not expressed a view on the legality or otherwise of the Iraq conflict," the spokesman said. "But that does not mean that individual members of the government should not express their individual views. These are long-held views of the deputy prime minister.

The legality of the Iraq war has long been a matter of dispute in British politics. On 30 January the then attorney general, Lord Goldsmith, wrote to Tony Blair, saying: "I remain of the view that the correct legal interpretation of [UN security council] resolution 1441 is that it does not authorize the use of military force without a further determination by the security council."

Goldsmith changed his mind on the eve of the invasion after Admiral Sir Michael Boyce, chief of the UK general staff, fearing troops might face legal action, demanded a clear ruling that the war would be lawful.

On February 16 2003, an estimated one million people marched through London in opposition to the invasion of Iraq. The then leader of the Liberal Democrats, Charles Kennedy, addressed the marchers in Hyde Park.

As Nick Clegg faced criticism at home, Prime Minister David Cameron was forced on the defensive during his Washington visit after describing Britain as the "junior partner" of the United States in the struggle against Nazism in 1940. In fact, as political opponents were quick to point out, the United States did not enter the European war until December 1941.