Polluting the River Orwell after oil seeped out from its Ipswich plant has cost a company £337,000.

One young swan died and 11 others survived after treatment following the leak which also threatened a site of international environmental and scientific importance, Ipswich magistrates heard.

This afternoon Eco Oil Ltd admitted charges of polluting the river and failing to comply with a condition of an environmental permit by not contacting the Environment Agency about the leak immediately.

The Canterbury-based company was ordered to pay fines and costs of £36,689.24.

The court had previously heard steps put in place to reduce the likelihood of a similar incident occurring and a hike in the company’s insurance premiums had already cost it around £300,000.

Environment Agency prosecutor Miriam Tordoff said the incident occurred around July 8, 2012.

The leak was traced to Eco Oil’s waste oil treatment plant on Cliff Quay.

It occurred after the plant was left unmanned between 7pm on July 7 and 5.54am on July 8.

Freak rainfall caused water to build up in an interceptor which should have prevented oil spilling out into the river. However, no one was there to regulate a control valve to prevent it doing so, despite heavy rain having been forecast, the court was told.

On the morning of July 8 a routine patrol of the river saw a heavy black oil slick in the water which was estimated as covering 4,000 feet by 60ft, and was two inches deep.

Mrs Tordoff said 800 metres downstream were several areas designated for special protection for birds and wildlife.

The harbourmaster made contact with the Environment Agency to alert it to the incident. However, a member of Eco Oil staff already knew about it, although the court was told he did not realise he should have called the Environment Agency as the harbourmaster had already done so.

Mrs Tordoff told the magistrates CCTV from Eco Oil showed that between 5am and 6am on July 8 black effluent was flowing overground towards an Anglian Water sewer outside the site entrance.

A total of 12 mute swans were subsequently taken to the RSPCA’s East Winch Rescue Centre in Norfolk for treatment, although one signet eventually died.

The court heard as well as failing to notify the Environment Agency, Eco Oil had not followed its own procedures by taking the proper precautions. In addition its alarm did not work and there had been an impact on the environment which would have been worse if it had not been for the efforts of the harbourmaster and his colleagues.

John Jolliffe, barrister for Eco Oil, told the court significant improvements had been made to systems costing £228,000. The company’s insurance premium had also gone up by £73,000 as a result of the £169,000 paid by its insurers for the clean-up operation.

In addition disciplinary action against staff and retraining had taken place.