A Second World War bomb has exploded unexpectedly in a Norfolk town, police said.

Emergency services and agencies declared a major incident following the discovery of the large unexploded device at a river crossing in Great Yarmouth on Tuesday, and had been working to disarm it.

On Friday afternoon, Norfolk Police said there was an “unplanned” detonation – but that no-one was hurt.

Army specialists had been cutting the bomb using a technique which creates a slow burn of the explosives, and burns off that material.

Officers had said there was a risk of an unintended detonation.

After the explosion, the force said on Twitter: “We can confirm the unexploded World War II bomb in Great Yarmouth has detonated.

“This was not a planned detonation & happened during slow burn work to disarm the explosives.

“All army & emergency service personnel are accounted for. We will bring you further info when we have it.”

The work to cut into it began on Thursday, but the water needed to do the work reduced the effectiveness of the sand barrier around the device.

The device is about one-metre (3.2ft) long and weighs about 250kg (39st 5lbs) – and was discovered by a contractor working on the third crossing over the River Yare.

There had been a 400-metre cordon at the scene.

Norfolk Constabulary’s Assistant Chief Constable Nick Davison said: “This has been a painstakingly long process but public safety and that of the people involved in the operation has been at the heart of decision making.

“This was the final phase of a delicate operation which has caused much disruption in the town, but we’re hopeful this could be resolved soon and that cordons can be lifted, if everything goes to plan.”

A spokesperson for Cadent, which manages the local gas mains network, said: “Our team has carried out a close inspection of our pipes in the vicinity. There is no indication that the blast has caused damage to our assets and gas continues to flow safely.”