Weston-super-Mare's Marine Lake is to remain closed after a swimmer got stuck in the mud.
Work has recently been carried out to flush out some of the silt in the lake so it could be used by swimmers throughout the summer.
The work used high tides to clear some of the mud ahead of a planned £300,000 revamp to mechanically remove thousands of tonnes of silt which is awaiting Government approval.
But the lake had to be closed on Thursday 22 July - a day after a swimmer needing rescuing from the water after getting trapped in the mud at the bottom of the lake.
North Somerset Council drained the lake following the incident, as they said hot weather had caused the water quality to deteriorate.
Following an assessment into the lake's conditions, the council confirmed it cannot be reopened safely.
North Somerset Council's executive member for neighbourhood and community services Mike Solomon watched the inspection, which saw volunteers and council workers get stuck in the silt.
"Last week's incident and the tests that we have conducted this week have demonstrated that the conditions have altered in the past four weeks since the lake re-opened," he said.
"Having seen the conditions just a few weeks ago compared with today I was shocked by the change. We simply can't risk people getting stuck in the silt. Public safety is paramount and therefore, for the time being, the lake must remain closed."
It is the volume of silt at the bottom of the lake which poses an issue.
The council as applied to the government's Marine Management Organisation to mechanically remove 30,000 tonnes of it and an outcome is expected in the next few months.
Matt Greatorex from Weston's Coastguard Rescue Team said: "The mud around our stretch of coastline can be extremely dangerous.
"With the closure of Marine Lake, we would urge people to take note of the warning signs, and avoid walking through the mud to try and reach the sea at low tide.
"Keep yourselves and your families safe, and if you see anyone in trouble or find yourself in difficulty on the beach or in the water, call 999 and ask for the Coastguard."