An unexploded bomb dug up on a building site was probably dredged out at sea and brought inland as building aggregate years ago, according to munition experts.

The two-foot long naval shell, six inches in diameter, was discovered by workmen clearing the old Charlie Brown's site in Otley Road, Shipley, on Monday.

The rusty shell was spotted in a pile of dumped earth scraped up in a diggers' bucket as builders were two weeks into a new development of three shops and 20 flats.

Police evacuated staff from the Royal Mail sorting office next to the site and Army bomb squad experts were called from Catterick Garrison.

It took them 30 minutes to make the area safe before packing the shell into a special sling to take it away.

An Army spokesman said the shell was taken back to Catterick where it was treated as live.

He added: "There was no fusing mechanism but it was unclear what was inside."

Royal Navy munition recovery expert Petty Officer (Diver) Paul Shaw, based in Portsmouth, said the shell - which would have been fired from cruisers or battleships in the second world war - could still have been highly explosive, but added it would have been in a "pretty stable" state.

"Over the years it has probably been rattled round in a dredgehead out at sea and the transported inland among other aggregate and built on. Nowadays aggregate is sifted through more thoroughly, years ago it wasn't."

He said live six inch shells could throw shrapnel "a good few hundred metres" and would have a devastating effect on anything it hit - especially people.

He said: "They don't like sitting around in lots of sunshine and getting too hot, although you could probably stand on it without anything going off.

"It would take quite a major knock to get it going, but it's better to play safe with them and leave them to the experts."

Petty Officer Shaw said the Royal Navy still recovered naval shells from the sea on an almost daily basis, but inland finds were more unusual.

"When they are found, 90 per cent of them have usually come from the sea as aggregate. The other possibility is that for some reason, someone put it there."

A spokesman for the Army also suggested the shell could have been left behind by a training corps during the second world war.

"The ships were all busy so they couldn't use them for training so they used to use fixed guns for training drills on land when they were recruiting personnel," he said.

Builders of Castleford construction firm Strategic Team Group were back on the Otley Road site yesterday, pushing ahead with the development that should take 52 weeks to complete.

Strategic Team Group's business development director, Charles Tweed, said: "We'd done risk assessments and all sorts before we started work on the site but we never expected to find anything like that.

"The Army's told us just to put anymore shells we find on one side and call the police but we're hoping that will have been be the last of them - we've got work to do."

e-mail: kathie.griffiths@bradford.newsquest.co.uk