The UK's biggest insurer has issued a warning to its home insurance customers that it will not cover them for chemical, biological or "dirty" nuclear attacks.

War risks are normally excluded. What we wanted to do was to highlight that David Ross, Norwich Union

Norwich Union, which covers one in five of all homes in Britain, said it had always had the exclusions in place on its home policies.

But it felt that it should inform policyholders of the exclusion following increased fears that the UK could be targeted by such methods.

It is thought to be the first insurer to issue a warning to policyholders. Other insurance companies are expected to follow suit.

Fears of attack

The insurer has issued the warning in renewal letters sent out since 1 January.

It tells policyholders that they would not be entitled to claim for damage to their property or its contents because of such attacks.

"Acts of war" or "terrorism" are traditionally excluded from insurance policies, but policyholders will still be entitled to claim for damage caused by a conventional bomb.

Businesses will be able to obtain cover for a chemical, biological or nuclear attack, but must pay additional premiums for the cover.

David Ross, a spokesman for Norwich Union, told BBC News Online that it felt it was "prudent" to make people aware of the existing exclusion, and said there was no change in policy.

"War risks are normally excluded. What we wanted to do was to highlight that, and to change the wording to make it clearer to our customers."

New policies, and those being renewed, are to contain a note stating there is no cover for "any act or acts including but not limited to: the use or threat of force and/or violence and/or harm or damage to life or property (or the threat of such harm or damage including, but not limited to, harm or damage by nuclear and/or chemical and/or biological and/or radiological means caused or occasioned by any person(s) or group(s) of persons, or so claimed, in whole or in part, political, religious, ideological or similar purposes."

War clause

Norwich Union is not the only insurer to take action in the face of a terror attack threat.

Last October Direct Line took the step of refusing to cover damage caused by biological and nuclear terrorism.

Previously, the insurer had only refused cover on the grounds of damage caused by an act of war.

A Direct Line spokesperson told BBC News Online that re-insurance firms had insisted on the change.