Forensic tests have been carried out on samples of the oil, which came ashore at locations across the South Hams, including Dawlish and Teignmouth, in November.

Some seabirds were killed after they came into contact with the large globules of the substance.

Experts at the Maritime and Coastguard Agency have said they believe it was old, weathered oil which could have been disturbed from the seabed.

Crude oil

At least 40 birds were found smothered in the oil when the slicks were discovered.

The South Devon Seabird Trust has said it is still caring for 22 birds which survived.

The Coastguard and Maritime Agency carried out tests on the substance which revealed it was a heavy crude oil which had been at sea for some time.

Another theory is the oil is from a tanker which illegally cleaned out its tanks in the English Channel.