Ondeo Industrial Solutions UK Ltd, which operate the effluent treatment plant at Ineos, were ordered to pay £3000 for the environmental breach last year.

Falkirk Sheriff Court heard that whilst Ondeo were carrying out maintenance work, last July, heavy rain increased the amount of water in the system.

Insufficient storage capacity meant the contaminated water was forced through the treatment plant, not allowing time for the oil to be removed and was discharged into the Forth.

The resultant spill stretched from the plant, over mud flats used by migratory birds, and all the way down to the Forth Bridges – nine miles away. No oiled birds or fish kills were reported.

Colin Bayes, director of environmental protection and improvement, at the Scottish Environmental Protection Agency (SEPA), said: "The area is of high nature conservation value and it is well recognised by a number of national and international special designations. It includes a large coastal area compromising a complex of estuaries, mudflats, rocky shorelines, beaches and salt marches.

"This successful prosecution highlights the serious nature of such polluting incidents which can have a devastating effect on a wide area of the eco-system."

Ondeo, based in Bo'ness Road, Grangemouth, pled guilty at Falkirk Sheriff Court, to carrying out a controlled activity liable to cause pollution of the water environment.

Fining the company, Sheriff Craig Caldwell, said: "The water discharged contained an excessive amount of oil.

"There was an aesthetic impact in that a slick was seen from the refinery to the bridges.

"Fortunately, there was no damage to the marine environment.

"The company operates in an industry that is of significant economic and environmental impact on the country.

"They take their duties and responsibilities very seriously and have co-operated with SEPA.

"Nonetheless, they have a duty to comply and should have had a contingency plan in place to deal with this event."

Grangemouth councillor Robert Spears was dismayed by the fine handed out.

He said: "The fact this company did not even have a contingency plan in place, really calls into question their ability to run this facility.

"This fine is a paltry amount and you wonder how this is supposed to act as a deterrent.

"SEPA also need to seriously think about having some kind of permanent office in the town, as these incidents and occurrences have been increasing over a long time."