The owner says the notice contradicts an EPA inspection.
The owner of a Westmeath adventure centre estimates a ''do not swim'' notice in Lough Ennell is costing him around €65,000 a year.
Frankie Wright, who runs Lilliput Adventure Centre, says the warning contradicts the results of checks carried out by the Environmental Protection Agency on the water quality.
His losses include a number of school tours which were cancelled after parents heard rumours of birds dying and cattle remains being found in the water - none of which are true.
Mr. Wright says continued mixed messages about the state of the lake are unacceptable: