Anglian Water is upgrading a treatment works to prevent "a potential danger to human health" .

Concerns over the safety of water supplies to customers in the King's Lynn and west Norfolk area emerged after AW announced it was starting work on a £4m revamp at its Gayton treatment plant.

A spokesman said: "An Anglian Water spokesperson said: “We identified the need to install an additional water treatment process at Gayton to ensure levels of nitrate remain well within the acceptable levels for drinking water in the future. We do this routinely as part of our planning processes, to ensure customers receive the highest quality drinking water.

“Once these proposals for the additional treatment have been approved by the Drinking Water Inspectorate, we enter a legal agreement with them which outlines specific timelines to ensure the work is delivered well before there would be any risk to the public water supply.”

Legal papers reveal the firm reported the potential for nitrate contamination from water supplied from the Gayton plant, which processes water abstracted from boreholes in November 2017.

A notice was issued the following year under the water quality regulations after AW said in a report "there is or has been a significant risk of supplying water in buildings, that could constitute a potential danger to human health".

AW was ordered to install an ion exchange plant at its Gayton works to remove the nitrates by December 2023.

It was also instructed to continue blending water with supplies from elsewhere in the system to reduce nitrate levels.

AW added: "There has been no impact on the water being supplied to customer and we continue to provide customers with the highest quality drinking water which remains perfectly safe to drink.

"The work we’re undertaking at Gayton water treatment works is a proactive scheme to ensure that we continue to meet the drinking water regulatory limit for nitrate in the final water at Gayton.

"As part of our submission for funding for water quality schemes, we highlighted the need for additional treatment to reduce nitrate concentrations at Gayton as we identified rising nitrate levels in the raw water sources. This is to ensure continued compliance with the regulatory nitrate limit."

The work is expected to take a year. AW was also ordered to carry out "an enhanced monitoring strategy for nitrate to include the monitoring of all high-risk water sources and associated final waters on a weekly basis to supplement the current regulatory treated water monitoring".

The notice warns failure to comply could result in legal action.