The pollution was reported shortly after 8am on Sunday, June 25 by sailors passing through the Menai Straits, the stretch of sea that separates the island of Anglesey from North Wales.
The straits are a particularly sensitive environmental area and the slick, believed to be a mix of light and heavy industrial oils, was originally thought to stretch from Port Dinorwie to Britannia Bridge, covering an area of four miles.
The Environment Agency later announced the slock was bigger than first expected, at up to eight miles long.
Holyhead Coastguard sent Bangor Coastguard Rescue team to investigate and informed the Counter Pollution and Salvage Officer of the MCA.
Initial investigations suggest the diesel came from the shore, rather than a vessel.
The authorities later identified the source of the pollution as an exhaust pipe at a derelict sewer works owned by North Wales police.
Barry Priddis, watch manager, Holyhead Maritime Rescue Co-ordination Centre, said: “We are currently working with the Environment Agency, the local authority and the Countryside Council for Wales in order to trace the source of pollution and endeavour to clean it up.
“We do believe that the pollution may be coming from a mainland source, although nothing has been confirmed as yet, we do have our MCA surveillance aircraft in the area which will help to identify the source of pollution, a Sea King from RAF Valley has also been assisting with surveying the area.
“We do not know if there is any risk to health but we would advise people not to go in the water, south of the Menai Straits.
He congratulated the yachtsmen for their quick thinking and informing the authorities about the problem.
We are pleased that the public are environmentally aware and welcome their reports on pollution,” said Mr Priddis.