But the drugs are just a precaution in case the submarine, HMS Torbay, starts to leak radiation.

First time

The potassium ioate pills prevent cancer of the thyroid - which is a large gland in the neck. The pills have been given to 31 schools within two kilometres of Portsmouth dock.

This is the first time that a precaution like this has been taken.

'Very unlikely'

Emergency planning officer for the council, Martyn Powell, said: "These pills would only be handed out in the very unlikely event of a leak.

"If there was a leak, alarms would sound and the schools and distribution centres would be alerted by phone."

A Ministry of Defence spokesman also said that a leak is very unlikely.

"There has never been an accident involving a Royal Navy nuclear-powered submarine which has led to the leaking of nuclear material into the atmosphere."