Mystery has surrounded the source of the oil which has led to more than 600 birds being swept ashore in Norfolk and Suffolk over recent days.

Experts from the Maritime Coastguard Agency have analysed the oil and found its state suggests it has been in the water for a very long time.

Shipping minister David Jamieson said: "What we know is that the oil in the sea is very heavily weathered, so it has obviously been there a long time.

David Jamieson: World War II theory for oil slick

"Some of these cases can be oil that is leaking out of ships as old as sinkings from World War II.

"Some of these ships take a long time to rust and the bunker oil from the tanks which fuelled the engines can take up to 60 years to leak from the tanks.

"Occasionally we get these leaks around our coasts. There used to be a lot of them in the western approaches to the Channel.

"These have largely died out now, but we do still encounter these cases where it can date back that far."

A Met Office spokesman said prevailing winds have switched from south-westerly to easterly and will remain in that direction over the coming days, leading to more oil being swept ashore.

He said: "It means anything on the sea surface is going to be pushed straight on to the Norfolk and Suffolk coast."