The constitutional expert Professor Vernon Bogdanor has argued that serving and former heads of the armed forces are becoming too vocal in their criticism of the government, threatening the constitutional convention by which the armed forces remain politically neutral.

Prof Bogdanor and Major General Patrick Cordingley, former commander of 7th Armoured Brigade and vice president of the UK National Defence Association, debate how far former commanders should be allowed to enter the public forum.

Prof Bogdanor told Today presenter James Naughtie that retired members of the armed service "have every right to take part in the political debate," but added that former personnel "must not appear to align themselves with a political party," particularly in the build up to a general election.

He said Lord Guthrie's criticisms of the prime minister's defence policies went, "beyond what is acceptable," adding that his comments "were unwise."

Major-General Cordingley explained that it was important for military experts to participate in the weapons debate, "how on earth do the public know what it is that they they should know about defence if people like Lord Guthrie don't have their say. And I think he didn't go far enough."