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Army experts working on a Second World War bomb have discovered nearby gas pipes - forcing them to delay their plans to safely detonate the "unstable" device.
Large parts of Great Yarmouth in Norfolk have been evacuated since the discovery on Tuesday of the 250kg, metre-long device.
Bomb disposal teams have since built huge blast barriers out of sandbags around the bomb, but have now stopped work on the next phase of the operation after finding two gas mains running under Southtown Road.
Norfolk Constabulary's Assistant Chief Constable Nick Davison said police would not take unnecessary risks, warning that the device "is unstable and should it detonate, poses a significant risk to life and property".
He said it was "painstaking work" and acknowledged it had caused disruption to people in the town.
"It became clear over night that the mainline gas pipes are in proximity to the bomb and therefore pose an additional, and potentially dangerous risk should the bomb detonate as [explosive ordnance disposal teams] attempt to disarm it and make it safe.
"This serious complication must be properly considered and evaluated before further direct work can be undertaken on the bomb."
Scientific experts from the Defence Science and Technical Laboratory – which advises the British Army on blast science - are working through options so that the authorities can decide on their next step.
Police had asked people to evacuate their homes and businesses, warning that there was a "real possibility" the bomb could explode.
They said anyone remaining within the 200m inner cordon around the site at Bollard Quay could suffer shrapnel injuries, and buildings could be damaged.
The bomb was found by a contractor working on the construction of a new river crossing late on Tuesday morning.
A major incident was declared by Norfolk's emergency services later that night, and rest centres set up for families asked to leave their homes.
He said bomb disposal teams would have to cut the bomb in order to do a safe controlled detonation, and had been building defences around the bomb.
"It's not often we find something of this magnitude right in the middle of our town, right in the middle of our community. This is unprecedented," he added.
How will army bomb disposal teams will detonate the Great Yarmouth bomb?
They have built a blast barrier barrier - to minimise the impact of any explosion - around the 250kg bomb since it was discovered on Tuesday;
Experts will then cut into the bomb which could take up to 12 hours before carrying out a controlled explosion;
If this is successful, the remainder of the bomb will be moved out to sea for a further controlled explosion.
An emergency helpline - 01493 330369 - has been set up for people who need help with evacuation from the exclusion zone.
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