Police have warned residents to stay indoors after a lion was reportedly spotted in fields in Essex.

The large cat, believed to be a lion, was seen in fields off Earls Hall Drive in St Oysth, near Clacton, Essex, just before 7pm yesterday, police said.

Officers from Essex Police have been working with experts from Colchester Zoo who believe the reports to be genuine after being shown a photograph from a member of the public.

However, police said the big cat had not escaped from the zoo, as all its animals had been accounted for.

There have been rumours that it may have escaped from a circus that had been performing in the Essex area, but they are yet to be substantiated.

"We are investigating where it may have came from but it is fairly safe to say that it did not escape from Colchester Zoo," a police spokeswoman said.

Two police helicopters using heat-seeking equipment have been involved in the hunt for the animal, while zoo workers said to be armed with tranquilliser guns have also joined armed officers.

Bernadette Cleere, whose aunt runs the King's Arms pub in St Oysth, told Sky News that when she first heard about the possibility that a lion was on the loose she thought it was "a hoax".

But after speaking to people who live in the area where the sighting took place, she is now taking it seriously.

She said: "We do know the people on the farm where it was reported. They have been told to be very cautious, stay in and keep their doors locked.

"They have helicopters and armed police around their house so I think they are quite scared."

She added: "There was a circus in Clacton about a week ago so some people are saying it has come from there, some are saying it has come from a zoo, but no one knows. It's not the thing you expect to hear - that there's a lion prowling around."

Che Kevlin, another resident, said he heard what he believed to be a "loud roar" at around 10pm while out walking his dog.

"It was worrying as we had just been for a walk with the dog. We saw the police helicopter but thought it was just searching for a person," he told the BBC.

Police have urged anyone who sees the animal to call 999.