Four former chiefs of the defence staff have attacked the prime minister's plans to let Parliament have the final say on sending troops to war.
The power to declare war currently lies with the PM
The ex-armed forces heads told the House of Lords that the proposal was "too academic" and "devoid of realism".
But the government said that in a democratic society it was right that elected MPs should decide.
Earlier, the MoD denied reports that training could be cut for 1,000 new recruits to meet personnel shortages.
Just days after he entered 10 Downing Street, Mr Brown announced proposals to reform the constitution - including giving MPs, rather than the prime minister alone, the right to decide on whether the UK declares war.
Under the government's plans, parliament would always have to be consulted in advance unless there "is not time" to get its consent or "there is a need for covert or secret operations", such as in rescue missions.
But the prime minister would be held accountable to MPs afterwards when "he had committed armed forces under exceptional circumstances".
One former chief of the defence staff, Lord Brammall, said he was enthusiastic about the idea, saying it would be in the "best interests of the country".
But General Lord Guthrie of Craigiebank - who was head of the forces under Tony Blair - said "secrecy, security and surprise" were critical to many military operations.
"Parliament's stamp of approval is important but parliament mustn't run the risk of hazarding the lives of service men and women," he said.
Field Marshal Lord Inge warned that it would be difficult for Parliament to be consulted when there was a need for speed - as there had been in Sierra Leone.
He said he was strongly in favour of the Commons debating any military deployments, but that if "there is any delay in it giving support to the armed forces, it is bound to affect planning and their morale."
Marshall of the RAF Lord Craig of Radley said that conflicts such as Iraq, where there is time for debate before launching military action, may not occur in future.
He asked: "How often in future are we going to embark on offensive wars of choice? Have we not had enough of them?"
Field Marshal Lord Vincent of Coleshill, said he understand how the invasion of Iraq had led to some "relevant concerns".
"But in today's less clear cut security environment this can be an immensely complicated matter which needs to be considered further before we completely abandon the royal prerogative or something equivalent," he added.
Lord Hunt of Kings Heath, for the government, said ministers would have to strike a balance to guarantee flexibility and forces' morale.
He said: "It seems axiomatic that in a democracy rooted in parliament, parliament should have the final say.
Earlier, the Times reported that training for some new Army recruits might drop from a minimum of 26 weeks to 14 weeks, to free up them for deployments earlier.
It added that the move was being considered because the battalions due to replace 52 Brigade in Afghanistan are at least 100 personnel short.
But the MoD said no-one was deployed on operations without the right training.
It added that it had no plans to change training for regular forces, but it was reviewing reserve forces' training.
The BBC's Caroline Wyatt said that the UK's military commitment to Afghanistan was not in any doubt, but that the forces are under-strength and increasingly stretched.