Levels of a toxic “forever chemical” rose to 13 times the legal limit during Thames Water pilots for a controversial multimillion pound water recycling scheme that will pump millions of litres of treated sewage into the River Thames during drought.

The data from the pilots is in stark contrast to public comments from Thames Water that their water recycling project in south-west London will not harm the riverine environment.

In technical notes in the company’s files for the project, seen by the Guardian, the results from two pilots reveal the recycling system rapidly and significantly increased levels of a particularly toxic type of Pfas chemical perfluorooctane sulfonate (Pfos), a suspected carcinogen – breaching the legal limits by as much as 13 times.