Swimming has been forbidden at four north Dublin beaches because the water has been contaminated by a suspected sewage discharge from a waste water treatment plant.

The beaches affected are Skerries south beach, Balbriggan front strand beach, Rush north beach and Loughshinny beach.

Fingal County Council issued the no bathing warning on Saturday and it will remain in place for three days.

Three of the four beaches which failed to achieve blue flag awards for high standards of environmental purity and management in May are located in areas which have insufficient sewage treatment.

Blue Flag beaches 2016

The blue flags are an internationally recognised symbol that beaches enjoy “excellent” quality bathing water as well as a range of facilities including information on the environment clearly displayed for visitors.

This year just four bathing places and marinas which entered the competition failed to achieve blue-flag status.

Three lost their flag for failure to meet excellent standards of water purity while the fourth, Ross Strand in Co Mayo, lost out due to there being less lifeguard cover than advised, during inspections.

The three which failed water quality tests were Portrane Beach at north county Dublin, Garretstown and Redbarn near Youghal in east Cork. They are all located in or near areas identified by the Environmental Protection Agency as having insufficient wastewater treatment facilities.

Portrane is next to Rush and Loughshinny – two areas which were identified in the Environmental Protection Agency Urban Wastewater Report as providing insufficient treatment.Similarly, Redbarn and Garretstown are close to Youghal in Co Cork which was also named by the Environmental Protection Agency as having insufficient wastewater treatment.

A spokeswoman for Irish Water said upgrades including a new treatment plant were being carried out to the sewerage network in Youghal. She said Irish Water plans to decommission the existing septic tank at Loughshinny and pump the waste water to the existing Barnageeragh wastewater treatment plant near Skerries. Contracts are expected to be signed later this year for a €7.3 million upgrade to the sewerage network in Rush, close to Portrane beach.

In all, an Taisce awarded 85 International Blue Flag and 56 National Green Coast Awards to Ireland’s beaches and marinas for 2016.