The reason why Prestonians couldn’t drink their water without boiling it for a month has finally been revealed.

United Utilities has been fined after a cryptosporidium outbreak at its Franklaw treatment plants to the north of Preston,

The Drinking Water Inspectorate found the problems came from the Franklaw works using a different reservoir to source water

Rainwater running off agricultural land was able to access an underground water tank at Barnacre.

A ‘planned change in operations’ allowed the entry of the contaminated water into the treatment process.

Traces of cryptosporidium were detected in the water at Franklaw triggering a shut off of supplies for 700,000 people across Lancashire.

Supplies for many were knocked out for a month during the summer of 2015 as engineers worked to fix the issue.

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At Preston Crown Court the hearing fined United Utilities £300,000 and additional costs of £150,000 were also agreed. The firm had pleaded guilty to supplying water unfit for human consumption.

Chief inspector of Drinking Water Marcus Rink said: “This prosecution was brought about because the company failed to follow nationally recognised and published good practice in assessing the risks of returning stored water to critical stages within the treatment works and to take appropriate and rapid action to protect consumers when the contamination was known.”

United Utilities was criticised for not acting fast enough to issue the boil water warning to households and businesses.

It has since paid out £20million in compensation to customers through reduced water bills.

Chief executive Steve Mogford said: “We are very sorry for the impact this had on our customers. I know from first-hand the inconvenience this incident caused, having lived in Lancashire for forty years. We have learned valuable lessons from what happened and have put technology and processes in place to guard against a repeat of this type of incident. United Utilities is now a leading company in terms of resilience to cryptosporidium.

“Public safety is always our primary concern and customers can be reassured that the North West’s drinking water is of an extremely high quality. Incidents of the kind that affected Lancashire in summer 2015 are thankfully extremely rare. The fact that we spotted the bug quickly, through our routine sampling, and immediately issued precautionary advice, minimised the risk of any customer falling ill.”

The fine was branded ‘pathetic’ by Preston MP Mark Hendrick

It is @unitedutilities who should apologise for allowing our water to be adulterated. £300,000 on a turnover of £1.7Bn is a pathetic fine. https://t.co/NXaFS0SOqI

— Mark Hendrick (@MarkHendrick_) October 10, 2017