A popular day-ticket carp complex is closed until further notice after more than 400 fish perished.
The fish, mainly carp, have died at Walthamstow No1, which is operated by Thames Water and is home to carp to well over 30lb.
There are seven carp venues on the site in East London and all have them have been temporarily closed since yesterday (24/5/17).
A spokesman for Thames Water said: "The fishery is closed as a precautionary measure after a number of carp in reservoir one had to be removed for a health check.
"Springtime mortality of fish is a fairly common natural occurrence but the number of fish removed has been significant enough for the Environment Agency and CEFAS to be informed in order to undertake a number of tests to ascertain the cause of death.
"Though the losses appear to be restricted to reservoir number one, the fisheries team feel it best to reduce stress on the other coarse-fishing reservoirs by imposing a site-wide closure until further notice.
"The fly fisheries on reservoirs four, five and East Warwick are not affected and will remain open as usual.
“We’re sorry for any inconvenience and appreciates the support given by our anglers at this time.”
An update is expected next week.