They have acted after a a series of incidents of people trespassing into the quarry to swim during the recent hot spell.
They have also repeated warnings of swimming in a lake situated at the site on Grovebury Road.
Aggregate Industries, which owns the sand quarry, says people risking their lives by treating the site like a beach. Groups have been spotted at the quarry, which is still in operation.
General manager Tom Wise said: “Swimming at a lake within a working quarry is extremely dangerous. Trespassers are risking their lives and it is not the first time this has happened.”
Six years ago, messages on social networking sites encouraged large groups to congregate at the quarry on more than one occasion. Mr Wise said there was evidence of groups of up to 50 or 60 people.
He added: “Behaviour like this could result in a serious accident or death. The sand banks are likely to be unstable and could collapse at any time. Swimmers have no knowledge of how deep the water is or what lies beneath the surface and could drown, or even be buried alive. It’s that serious.”
The quarry has signposted warnings around the site, encouraging people to stick to the footpaths. Boundary fences, a legal requirement, are in place to stop people wandering on to the operational parts of the quarry.
Security guards are in place to man the perimeter. However, increasingly, they are having to stray from their main duties and instead redirect members of the public back to the nearest footpath.
Mr Wise said: “We understand that when the sun is shining, people want to get outside and enjoy everything the great outdoors has to offer. However, we are asking that members of the public heed our advice and stay safe.”
Round the clock security patrols are now in place and Aggregate Industries says those caught trespassing on the quarry could face prosecution.