In the worst cases, submarines have been repeatedly attacked with explosives to recover valuable scrap and metal alloys. Propellers, made from high- quality bronze, are particularly sought after.
As Remembrance Day once again focuses attention on memorials to the war dead, most divers condemn such actions. But they also point to confusion and some hypocrisy in the official position, which in the past has sanctioned commercial salvage on vessels where hundreds of sailors died.
Marine archaeologists and other conservationists are worried that important parts of Britain's maritime heritage are being lost for lack of a concerted policy.
By contrast, an initiative was launched by English Heritage this week to preserve the 50,000 to 60,000 war memorials that dot the UK.
Some of the recent abuses are highlighted in a BBC investigation, to be broadcast on BBC1 tonight. The programme, Close-Up North, shows howrecreational divers have been plundering a number of German submarines sunk off the North Sea coast in both theworld wars.
One scene shows divers from Brighton landing in Scarborough two months ago, carrying armfuls of relics recovered from war graves of the Battle of Jutland, which occurred on 31 May, 1916.
Shots taken inside Scarborough's Sub Aqua club bar show a U-boat periscope removed from a wreck found off the coast. U-Boat UB107 sank during a Royal Navy attack in the First World War. It went down with all hands.
One of the Brighton divers, who had salvaged shell cases and other artefacts, reflected the confusion of the situation when interviewed for the programme. "They're what we would call trinkets," he said. "They'll fit on the mantelpiece. They've got a bit of history with them, which is why you pick bits up.
"You've got to respect - a lot of people have died on some of these wrecks. Basically, if people have died you're not allowed to rip them apart and basically rob the dead."
The example of UC75, sunk off Flamborough Head, Humberside, shows that some rogue divers completely disregard even this interpretation of the rules.
The boat originally contained the remains of 20 German sailors, but it was blown apart after news leaked of the location of its undisturbed final site. Salvors attacked it with gelignite and removed the propeller, torpedo tubes and deck gun, reportedly to sell as scrap.
The wreck was first located by a Hull diver, Barry Thompson, who officially notified the German government. He told the BBC investigation for Close- Up North: "When I found it, it was in pristine condition, covered in anemones - it was a majestic site. After other divers discovered the site, it became a different wreck. It's been swept clean."
A fellow diver, Andrew Jackson, agrees. "Down the coast there are divers who are, basically, just animals," he said. "They will just blow these things to pieces to get inside them to plunder them."
He gives the UC70 at Whitby as another example. "The conning tower is hanging off that and it's got big holes where explosives have been used to get inside it," he said. "The propellers have been removed, the periscopes have gone.
"You know it's just totally plundered and it's just defiled effectively, if that's the right word."
Spoils prised from sunken submarines are not only found in private houses or in enthusiasts' collections. Another submarine propeller, from UC709, is on public display at a museum in Bridlington.
Of the 24 submarines in the North Sea whose sites have been located, six are known to have been pillaged.
The situation has provoked much anger among German U-boat veterans.
Horst Bredow, of the U-boat Archive in Cuxhaven, said: "When there are bodies on a ship it is a sea grave. That is recognised by every civilised country in the world. They should not be touched.
"We have heard of what is happening and it is very sad but there is little that we can do about it."
Technically, all items recovered from the seabed should be handed over to the Receiver of Wreck. But this rule proves hard to enforce. Divers were once policed by dozens of officers based in most major ports, but now their jobs have been taken over by one person.
Veronica Robbins, Britain's new Receiver of Wreck, said that she knew of the UC75 but was unaware it had been blown apart. She said: "We are not aware of materials being taken from this wreck. It is an offence to intrusively dive a war grave."
Under the 1986 Protection of Military Remains Act, the Secretary of State for Defence has the power to make it illegal to dive on a wreck by officially designating it as a war grave.
But so far, not one wreck in the North Sea has been designated, meaning both British Naval and German wrecks have no formal protection.
Only two sites have been covered by law. One is the remains of HMS Royal Oak, sunk in 1939 at Scapa Flow in the Orkneys. The other is the site where HMS Sheffield lies off the Falkland Islands.
But dive operators working in Scapa Flow, now a popular recreational area, point out there is another wreck there, the HMS Vanguard, which was sunk in the First World War. While it is now called a war grave by the Royal Navy, and any diving is forbidden within 100 metres, it was licensed for commercial salvage until the early Eighties and its propellers and boiler fittings have been removed.
"The Ministry of Defence just seem to make up their own rules for whatever they want," said John Thornton of Scapa Flow Technical, a dive centre. "If they want the propellers off a ship, they take them and then stop anyone going near.
"As for divers, I think it's just like a group of people going shopping - 90 per cent behave themselves but some will go shop-lifting. Yes, some people do take stuff, but most respect the law."
Clarification of that law - and more effective policing - would mean another part of Britain's wartime heritage was more secure for future generations.Reuse content