Tougher rules are needed to prevent licensed firearms being used in fatal shootings, the head of the Independent Police Complaints Commission says.
The IPCC is investigating the shooting of Pc Richard Gray
Nick Hardwick is due to raise the issue in a speech at the conference of the Police Federation of England and Wales.
He is expected to express concern over about 12 suicides or murders where licences should have been revoked.
The IPCC is looking into a shooting in Shrewsbury where an officer was killed by a man with a legally-held weapon.
Peter Medlicott is believed to have shot Pc Richard Gray with a .222 calibre rifle which he used for pest control work, before turning the firearm on himself.
The officer was confronted after being called out to a domestic disturbance on 6 May.
Mr Medlicott had been arrested for assault on 10 April.
Mr Hardwick is expected to tell delegates in Blackpool: "We are concerned about the number of fatal shooting incidents - suicides or murders, often with a shotgun - where there do seem to have been warning signs before the incident that suggested the firearms licences should have been revoked."
He will add that a firearms licence is a "privilege, not a right".
"If causes for concern are identified - such as violent incidents or mental health problems - we need to see if more stringent arrangements should be in place to decide whether the licence - and the firearm - should be reviewed," he will say.