Britain's best fishmonger's, and much-loved tearooms, were victims of a mystery blaze which swept through shops and derelict buildings in Wellington yesterday.

Although the fire, in Cornhill, is still under investigation, there was speculation that intruders accidentally or deliberately set fire to rubbish in one of the derelict buildings may have started the inferno.

Avon and Somerset Police and Devon & Somerset Fire and Rescue Service have launched a joint investigation, but checks at the source are on hold after building inspectors declared the derelict building unsafe and shut Cornhill.

The fire is thought to have started shortly after midnight and spread quickly from the ground floor to the roof and into an adjoining building housing Don Jones's fish shop and Cornhill tearooms. It also spread to an empty building on the other side, but dozens of firefighters from Somerset and Devon managed to stop the flames reaching five other properties in the row.

The alarm was raised by Sam Cunningham, the fish shop manager, who had moved into a flat over the shop just before Christmas specifically to deter vandals attacking the empty buildings. Smoke alarms in the flat alerted him just after midnight and he managed to escape.

Paul Godwin of Devon & Somerset Fire & Rescue said: "It is very important that people maintain their smoke alarms. I would hate to think what could have happened here if they had not operated. This was a challenging fire.

"We had difficulty in accessing the rear of the properties, and it's a very narrow pedestrian walkway at the front. To reach the rear we had to break through an adjoining unoccupied building."

Crews from Wellington, Taunton, Bridgwater, Nether Stowey, Burnham-on-Sea, Minehead, Wiveliscombe, Bampton, Tiverton and Cullompton were called to fight the blaze. They are also understood to have rescued three pet hedgehogs owned by Mr Cunningham.

Don Jones's fish shop was founded in 1851 and last year won the Seafood Independent Retailer of the Year award.

Steven Freeman, who has run the shop for the last 22 years said: "We are the best fishmonger's in Britain and to see this is very distressing. Sam moved into the flat just before Christmas because people had been seen around the buildings and we thought that if they saw a light on above our shop it would make a difference.

"The fire started in a building which has been empty for a long time and has no working electrics in it. I believe a fire officer had notified the authorities of the potential problem."

There has been talk of regenerating the ancient narrow street for many years and Taunton Deane Borough Council and Wellington Town Council set up a joint task group. Then the owner of much of the property put in a planning application, but flats which were part of the scheme were deemed out of character.

Yesterday Councillor Andy Govier said: "It is imperative that it gets going again. It's a real shame that two such successful businesses should suffer like this. In the short term we must look at whatever other ways we can help."