More than 15 tonnes of the road building material has already been removed from the water after the spill last month.

The incident happened at FM Conway’s bitumen storage facility at Imperial Wharf in Gravesend where the material is shipped-in and stored before being distributed to the contractor’s highways sites across the UK.

The site also houses FM Conway’s in-house Technology Centre and laboratory where new bituminous products are designed and tested.

A statement by the Port of London Authority said: “On 17 August, FM Conway, a commercial operator working from Imperial Wharf in Gravesend accidently released bitumen – an oil product used in construction projects and to pave roads – into the River Thames.

“Since then, the Port of London Authority has been using its expertise to coordinate the clean-up operation working with partners across the region. So far, we have removed around 15 tonnes of bitumen from the River Thames. We will continue our efforts to search for, contain, and remove the spill.

“If people find what they suspect to be bitumen, please don’t try to remove it yourself but report it to your local council.

“Our expert contractors advise that it is not harmful to human health, but it requires proper handling and disposal to avoid further contamination. The Environment Agency will lead the investigation into the cause of this incident.”