It is five years since a sound that had become part of weekly life in Walthamstow was heard for the last time.

The siren at Coppermills water treatment works, which has sounded once a week for the past 40 years, will ring out across Walthamstow for the last time this week.

A £37 million upgrade to the Thames Water plant, in Coppermill Lane, has left the air raid-style siren redundant. It will be tested for a final time on Saturday, May 7.

The alarm has been tested every Saturday at noon since the 1970s, but has never been used for its real purpose – to alert those within a mile of the site of an accidental release of chlorine gas.

The use of chlorine gas at the plant, has been phased out in favour of safer liquid chemicals.

Natasha Coackley, manager of the upgrade project, said: “The upgrade began in 2012 and formed one of our largest programmes in recent years.

“Many of our processes at Coppermills were obsolete and we took the opportunity to replace the old gas disinfection plant.”

The treatment works, which is one of the major water processors for the London area, first opened in 1969.

It was originally owned by the East London Waterworks Company, before later being purchased by Thames Water.

David Knight, Thames Water health and safety specialist, added: “Getting rid of chlorine gas from Coppermills is the biggest change to community safety we’ve ever done.”

Residents living close to the plant were informed weekly siren test would end in a letter sent out last week.