DEFRA has been unable to identify the source of the highly pathogenic H7N7 virus in the 4 June outbreak on a free range layer unit near Banbury, Oxfordshire.

In its final epidemiology report published today, DEFRA concluded that "the infection was confined to a single premise and there was no evidence of infection on any contact or geographically close premises, or evidence of spread of infection to any other premises to date."

The two most likely sources of the outbreak are thought to be unidentified avian flu in a domestic flock in Great Britain, associated either by proximity or potential contact, or it came from wildlife.

Deputy chief veterinary officer, Alick Simmons, said: "This incident has demonstrated again the potential for avian flu to be introduced into domestic poultry in the UK. The risk of further incidents, while low, remains."

He urged poultry keepers to maintain the highest standards of biosecurity and to report suspicion of disease promptly including where records indicate increased mortality or reduced performance."

Surveillance Zone and remaining disease control area restrictions were lifted earlier this week (Tuesday).