Jill Marshall-Andrews, chairwoman of the Gun Control Network, said: "We are dismayed at the prospect of more and more children and young people being trained in firearms.
"In our view the age limit for use of guns should be 18 and it is quite inappropriate to introduce anyone younger to dangerous weapons."
The Secretary of State for Defence, Michael Portillo, defended the plan. "The armed forces save lives. They don't lose them and the young people will learn discipline and hard work when they join as well."
To deflect the criticism, ministers are expected to concentrate on plans to give the young recruits self-defence training and orienteering skills. But weapons training could be part of the scheme.
Conservative Party sources confirmed the plan had Mr Major's backing, and was likely to be on the agenda at a meeting of the Cabinet on Monday to agree the Tory election manifesto. But the plan is likely to be announced next month.
Mr Portillo said: "The cadets are a good thing because they help young people to develop self-discipline, self-motivation and qualities of leadership. They make young people team players, give them self-esteem and satisfy their love of adventure.
"For some young people, they point the way to a career in the military; for others, they simply lead to more knowledge about themselves."
The MoD has a pounds 68m budget for the cadets, and a big expansion of the scheme to the state sector would mean a substantial increase in costs, which has yet to be approved.
However, it has won the support of Michael Howard, the Home Secretary, and Gillian Shephard, the Secretary of State for Education and Employment.
Brian Mackenzie, of the Police Superintendents Association, said the Government would have to exercise caution before giving children weapons training: "Young boys and girls will get a sense of belonging. But in the present climate of opinion, they will have to exercise extreme caution before teaching these kids to use rifles."
A total of 198 public schools and 45 state schools run cadets for the three main services, and the more elite Combined Cadet Force. The MoD is seeking to expand the voluntary scheme into more state schools.
The aim is to instill discipline into young people by recruiting them to the cadet forces for the RAF, navy and army with the prospect of outward bound exercises and camping holidays abroad. The Government will pay for their uniforms, but they will have to pay for their boots.Reuse content