All staff, from sailors to office workers, under the age of 50 will undergo compulsory annual fitness tests from early 1999.
Anyone who fails to meet a minimum standard will be put on a remedial programme and re-tested a few months afterwards. Those consistently unable to slim down will be dismissed.
Ailsa McIntyre, a spokeswoman for the Royal Navy at the Ministry of Defence, said: "This new policy will provide better assurances that people can undertake operational tasks... It brings the Navy into line with the Army, RAF and Marines."