Armed police have been deployed after a member of the public reported seeing a lion in a field close to the village of St Osyth near Clacton in Essex.
Essex police confirmed to ITV News that firearms officers and a helicopter had been deployed to the area.
A statement on the Essex Police website said:
Experts from Colchester Zoo said they believe the report is genuine after looking at a photograph sent in by a member of the public.
It is not yet known where the lion escaped from, although all of Colchester Zoo's animals have been accounted for.
Dave Sparks runs the Red Lion pub in Essex, near where the lion was reportedly spotted. He says his evening has been "quite surreal" but he did not feel like he was in any real danger.