Coppers in Oldsmar, Florida, said the hacker used a remote access programme used by the treatment plant workers to increase the amount of sodium hydroxide by a factor of one hundred, from 100 parts per million to 11,100 parts per million.

Normally this chemical is used to treat water acidity but the compound is also found in cleaning supplies such as soaps and drain cleaners. It can cause irritation, burns and other complications in larger quantities.

The hack was noticed by a supervisor who spotted a cursor moving across a screen and reversed the action in time.

Pinellas County Sheriff Bob Gualtieri said during a news conference on Monday that the public was never in danger, but that the hacker did raise “the sodium hydroxide up to dangerous levels”.

Officials in the suburb of the city of Tampa have since disabled the remote-access system, and say other safeguards were in place to prevent an increased chemical concentration from getting into the water.

Experts say municipal water and other systems have the potential to be easy targets for hackers because local governments’ computer infrastructure tends to be underfunded because Americans think the state spending money on water plants is communism.