The mix of bad luck, explosives and carelessness in the face of extreme danger would have caught the attention of writers in the heyday of Ealing comedies.

Picture the scene: a large second world war bomb washes up on an English beach, causes panic and the evacuation of a seaside town before it is towed out to sea for safe detonation. But the relief is short lived. The bomb detaches from its leash and is lost, unexploded.

This is how the 1,000lb German SC shell, which washed on to Felixstowe beach last week, has come to be currently lost off the coast. The 1942 bomb is thought to be one of the largest ever to have beached on Britain's coastline with an explosive strength sufficient to flatten parts of Felixstowe's seafront.

Today, Royal navy divers admitted it could take weeks to find the "misplaced" bomb.

The bomb was towed out to sea last Tuesday, with the intention of detonating it the same day, but conditions delayed the operation. Strong currents moved the bomb's position overnight, and the straps connecting it to the ship broke.

Officers said no global positioning system device had been attached to the bomb.

Divers had been confident the bomb was within a 200-metre perimeter of where they originally dropped it but have now scoured four other similar-sized areas to no avail.

The 13-strong team are now using an unmanned computer-operated submarine called Remus to picture the seabed to locate suspicious objects for their dives. The 1.72m (5ft 8in) torpedo-shaped device was driven down to Felixstowe from Scotland, while the divers themselves made the trip from the south coast.

"The contacts we have been finding with the Remus equipment have proved not to be the bomb," said Lieutenant Commander Mark Hankey.

"The good news is the equipment is working and we are finding a variety of things and able to look at this stuff and we are now widening the search area.

"We are doing this as diligently as we can and this takes time. The main thing is this bomb is off the beach and away from an area where it can do significant damage or harm."

Warrant Officer Robin Rickard, who is leading the dive team, said they were now working 15-hour days in an attempt to finish the job.

Conditions were still said to be hampering the operation with underwater visibility at zero and strong tidal streams meaning the bomb could have moved a "significant" distance.

"We will continue to search until we are told not to," said Hankey. "That is an operational requirement and if another job comes up and we have been here a month then we might have to say the bomb has rolled out to sea. But we will continue to search for the foreseeable future."

He added: "We may be here days, weeks - it is a decision that has not been looked at yet."

The navy team is staying at a hotel in the town while the recovery work is carried out and the overall cost of the operation is already believed to have cost tens of thousands of pounds.

Last night, the team said money had not been wasted and insisted the bomb would be found and dealt with, however long it took.

Hankey said: "What you have to bear in mind is the navy is paid for whatever they are doing. At the end of the day the only expense going on here as I see it is travel and subsistence - they are in a hotel."

Experts from the Royal navy's bomb disposal unit said the bomb had the potential to "flatten" a huge area of the town's seafront if it exploded on the beach - and cause collateral damage up to half a mile inland.