Thousands of litres of cooking oil were dumped into the Grand Union Canal in what is thought to be a deliberate act.
A major clean-up operation of the canal was launched near Little Venice in what has been rated as a Category 3 incident by the Environment Agency.
The source of the oil is thought to be a takeaway kitchen, six miles up the canal in Alperton, Wembley.
The incident has cost the Canal and River Trust tens of thousands of pounds. The charity has said it is unlikely the polluter will be made to pay.
Pictures showed contractors using booms to gather the oil and pump it out of the Paddington Basin.
The charity is continuing to monitor the impact on water quality and wildlife. It urges anyone with information to report it to the Environment Agency.
Although the ‘worst’ of the oil will be removed from the water, scientists have advised that the remainder will naturally break down over the next few weeks.
A spokesperson for the Canal and River Trust said: ‘Our team and contractors have been working to tackle the pollution at the suspected source in Alperton, West London, and as far as six miles along the canal at the Paddington Basin, using booms to gather the oil and pump it out.
‘The operation has cost our charity tens of thousands of pounds and we urge anyone with information about the source of the pollution to report it to the Environment Agency so that the culprit is identified.
‘Our ecologists and environmental scientists continue to monitor the impact on water quality and wildlife. While we have been doing our best to remove the worst of the oil, we have been advised that the remainder will break down naturally over the next few weeks.’