Welwyn Garden City’s Stanborough Lakes will remain closed “for the foreseeable future” after a fish-killing disease was discovered.
Swimming and fishing had previously been suspended as "a precaution" after blue-green algae changed the water's colour, following which further tests were taken.
However, a message on Stanborough Lakes Fishery's Facebook page read: "Stanborough Lakes will remain closed for the foreseeable future as we have today [Wednesday, October 2] had confirmation from CEFAS that the lake and the carp have contracted the KHV virus.
"We will be meeting with a member of CEFAS next week to see how we can move forward from this."
KHV, also known as Koi herpesvirus, is a serious viral disease that affects all varieties of common and ornamental carp, and can result in large-scale mortalities, according to the government's website. The disease does not affect humans.
A spokesman for Better, which runs the lakes, said: "Last week, in consultation with the Environment Agency, we suspended angling at Stanborough Lakes to reduce stress on fish stocks caused by the dry weather.
"This week CEFAS have also issued a suspension of angling due to the presence of KHV.
"We are working with them on control measures and it is our intention to reopen the lakes for anglers as soon as possible."
"We will provide an update on social media.
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