Trespassers have been spotted swimming and jumping into and "highly toxic" waters on the site of former quarry in Chinnor, Oxfordshire.
The site's owner Taylor Wimpey said it had deployed security guards with dogs to patrol the site, but a spokesperson admitted to the BBC that people were "illegally accessing" the land and said it was "carrying out daily fence repairs".
Signs at the site in Chinnor, Oxfordshire, read: 'Do not enter water due to high pH levels. This can cause skin and eye irritations, stomach problems, fungal infections such as thrush, other infections such as rashes. Think! Would you swim in ammonia (pH 11.5) or bleach pH (12.6)? Well these are similar to the Blue Lagoon – pH 11.3)!'
Locals also raised the alarm, telling Henley MP John Howell about antisocial behaviour and parking obstructions near the Old Kiln Lakes.
Mr Howell said "gangs" had visited the fenced off area of the site "without any concern for social distancing".
Known to locals as the "blue lagoon" or "Chinnor Riviera", the quarry has steep edges and deep water in its fenced-off section.
Thames Valley Police warned the water, which contains highly alkaline substances, was "highly toxic and can cause serious health issues". A police spokesperson added: "A Section 34 dispersal order is in place and will be enforced where necessary to tackle anti-social behaviour," the spokesman said.