Palm oil is a type of oil derived from palm trees which is used in cooking, soap and biofuels.
There have been several cargo incidents off the north west coast over recent decades and it is estimated that tonnes of palm oil remain in the wreckages.
In addition, under the current regulations, ships are allowed to clean their tanks after delivering the substance to the UK, dumping a limited amount of the contaminated residue at sea, provided they do so more than 12 miles from the shore.