A number of Royal Navy sailors have been rushed to hospital after drinking contaminated water on HMS Portland.
The frigate diverted to Portsmouth Naval Base today "as a precautionary measure" due to an issue resulting from one of the warship's fresh water systems.
An investigation has been launched, with safeguarding measures put in place.
According to reports, a sailor had initially noticed something was wrong and flagged it to superiors - but it's not clear how many personnel are sick.
A Ministry of Defence spokesman confirmed the contamination to the Mirror.
It has also since been confirmed a "small number" of personnel were taken to hospital and HMS Richmond "has been stood up to cover any contingencies".
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An official Royal Navy statement said: “We can confirm that HMS PORTLAND has returned to HMNB Portsmouth as a precautionary measure, following an issue with one of the ship’s fresh water systems.
"The health and safety of our personnel is of the utmost importance and we are taking a number of measures to safeguard the ship’s company whilst the issue is investigated.
“A small number of personnel were taken to hospital as a precaution and HMS Richmond has been stood up to cover any contingencies.”
A source told the Daily Mail sailors were being rushed to Queen Alexander Hospital in Portsmouth.
They said: "The situation is quite serious. The ship has done everything they should do. They found out about it this morning and came alongside in Portsmouth.
"They have got scientists on board investigating. The water has been contaminated by something. A number of personnel have been taken to Queen Alexandra Hospital for treatment."
HMS Portland departed from port on January 7.
