The BBC's Ian Cundell reports on the Luftwaffe's legacy in Britain

The Luftwaffe dropped thousands of bombs between 1940 and 1945 on key cities such as London, Birmingham, Portsmouth, Plymouth, Coventry and Cardiff.

At least 600 have been deliberately abandoned by the Home Office, some of which are close to new housing or industrial estates.

British bomb fuses differ slightly from German ones

Ordnance experts say chemical changes in the last 60 years will have made the bombs more volatile than when they were dropped.

The oxidation of picric acid, which was used in the German fuses, has made them vulnerable to vibrations.

The construction union Ucatt is worried about the dangers to its members from bombs under potential building sites.

Capt Quinn: "We have to be very wary"

Captain Damian Quinn, a bomb disposal expert with the Royal Engineers, says: "We have to be very, very wary to the extent that a slight jolt or jarring could result in enough friction or enough shock to set the fuses off."

Thousands of British and American bombs were dropped on Germany during the war.

Three people were killed and 20 injured when a 55-year-old device exploded in Berlin earlier this year.

Two wartime devices explode in Hamburg every year and the law insists developers screen building sites for bombs before starting any ground work.

Experts try to defuse a bomb in Hamburg

Peter Voss, of the Hamburg Fire Brigade, says Britain could be sitting on a time-bomb. He says: "There are lots of bombs in Britain and it is preferable to start looking for them now."

One of the bombs is believed to be buried close to a housing estate in Hull, which was targeted because of its port facilities.

Alan Johnson, Labour MP for Hull West and Hessle, says: "No MP wants a bomb under their constituency and I'm particularly concerned for the safety of my constituents.

"I think this does require some investigation and reappraisal by the Home Office."

Alan Johnson MP: Constiituency sitting on a bomb

After years of secrecy the Home Office has now agreed to release details of the abandoned bombs.

A Home Office spokesman warned that the document in question was only an "inaccurate register of bombs dropped during the war".

He said the Home Office was currently working on how to put the information in the public domain and he said it may use the Internet to disseminate it.

The Lost Enemy, part of BBC Two's Close Up North series, can be seen by viewers in Yorkshire, Humberside and Lincolnshire.