Waterways Ireland has issued a "do not swim" notice for Clones Canal and Basin.
It is advised that swimming or partaking in water sports activities may cause serious illness.
Any activity is prohibited due to high levels of Intestinal Enterococci (bacteria).
Waterways Ireland says that action is being taken to rectify the situation and resampling will be carried out.
The notice will remain in place until water quality returns to the acceptable levels.
For further information contact Waterways Ireland on +44 28 6632 3004.