A hunt is today being stepped up to find the arsonists who destroyed a former hospital site a few miles outside of the city.

About 70 firefighters were involved in tackling the blaze which broke out at the Little Plumstead Hospital, to the east of the city, in the early hours of Sunday morning.

Investigations have confirmed arson to be the likely cause of the fire which gutted the hospital's Old Hall and resulted in most of it having to be knocked down.

A police spokesman said that while investigators 'have not been able to confirm the cause of the fire', it is believed to have been started deliberately and have issued an appeal for witnesses.

David Ashworth, assistant chief fire officer, said the investigation was hindered by the fact they could not gain access to the building due to the floor and roof structure being unsafe.

He said: 'We couldn't gain access to it to pinpoint exactly where the seat of the fire was.'

But despite not having been able to find where the fire started, Mr Ashworth said the building had been empty and secured for a number of years and had no electricity or gas connections, meaning that any ignition would have 'had to have been started by a person or persons'.

He added there was little that could be done to save the building which was due to be converted into flats as part of a housing development.

Mr Ashworth said: 'By the time we got here it was fully developed.

'We had to just make sure it wasn't going to spread to any other buildings and that the crews were safe and the people in the local vicinity were safe.'

He added: 'It's a loss from an architectural perspective. It was very ornate and intricate. It's sad to see it being lost.'

Fire crews have now left the scene, with what little that remains of the building, which dated back to 1889, fenced off to try and stop people from clambering through the charred rubble.

A spokesman for Broadland District Council said the building was not listed but was deemed 'worthy of retention' by the council and was to be converted into flats as part of the approved planning application.

The spokesman said if the whole building has been destroyed it would need to come back to planning before anything could be built as permission was granted on the basis the building was to be converted.

No-one from Beccles-based Cripps Developments, which owns the site, was available for comment last night.

As reported last year, buildings at the former hospital site were to be demolished – bar the Old Hall which will be converted – to make way for 119 homes.

About 300 homes have been built in the Hospital Road area since 2000 and the Little Plumstead Primary School relocated there in 2011.

Witnesses should contact Det Con Richard Long at Norwich CID on 101 or call Crimestoppers anonymously on 0800 555 111 or via their anonymous online form at www.crimestoppers-uk.org.