Standing by Geoff Hoon, the defence secretary, at a joint press conference called to discuss today's Nato summit in Prague, he rammed home the point that he advised ministers that the armed forces should not cross picket lines.
Military commanders were furious last week when ministers, notably Lord Falconer, the Home Office minister, raised the stakes by threatening to send in troops to take control of modern fire engines.
Such a decision would have serious constitutional and legal implications, senior defence sources said. "Huge political stakes are involved," said one senior defence source.
Yesterday, Sir Michael - who is to leave his post early next year, though he expected to be asked to stay on - did not pull his punches. He said the provision of cover for firefighters was having a serious effect on the armed forces, which were already stretched to the limit.
He was "extremely concerned" about the "military effectiveness" of the armed forces and about the "impact on individuals".
He added: "Clearly we don't have a box of 19,000 people standing by to be called upon to do firefighting duties. They must have been drawn from operational units".
Sir Michael explained that those on standby for firefighting duty were not in training for any coming conflict.
But Sir Michael did not leave it at that. "There is also the problem," he said, "of impact on individuals because a lot of people have come straight off deployment from Bosnia, from Afghanistan or whatever and gone straight on to these duties without the benefit of any home time and so there is a morale and motivation problem to be addressed as well".
Military commanders have had to tie up ships, take vital fire crews away from RAF stations, and stop exercises, as a result of the firefighters' action.
Under Operation Fresco, the military's codename for their cover, armed forces personnel have 12-hour shifts equipped with vintage green goddesses.
Military chiefs are rigorously neutral about the dispute, as they have to be, at least in public. Their concern is compounded by increasing frustration about Mr Blair's failure to take what they call a "political decision" to mobilise British troops for a military campaign against Iraq.
Sir Michael and his fellow chiefs of staff expected clear orders by last month. Though they have drawn up contingency plans involving some 15,000 troops supported by navy and airforce personnel, they still do not know what to prepare. Without a green light from Mr Blair - and Gordon Brown - they cannot modify tanks and other equipment for the desert.
Mr Hoon disclosed yesterday that he had received a request from Washington for forces for an Iraqi campaign but added that the government had still not taken a decision. Though he gave the impression he would finally announce the deployment of British troops - and reserves - on Monday, defence officials later damped down any expectations leaving the prospect of continuing uncertainty.
US officials have said privately that they want the support of British special forces, mid-air refuelling tankers and reconnaissance aircraft. But the government is likely to want a more visible British military presence - including battletanks - to reflect its support for the Bush administration's approach towards Saddam Hussein.
Mr Hoon interrupted Sir Michael more than once, insisting that the armed forces could both cover for the firefighters' strike and help to provide what he called a "credible threat" to Saddam Hussein. After Sir Michael again emphasised the impact on the morale and motivation of the troops of the firefighters' action, Mr Hoon intervened to play down any suggestion that the armed forces have problems or stretched too thinly.
The relations between the two have been strained since the September 11 attacks on the US when Sir Michael openly criticised some of the more hawkish rhetoric coming out of Washington. He also expressed scepticism about President Bush's war on terror.
