Mr Mangnall, the Conservative MP for Totnes – which also covers Brixham, said anger around the town was “palpable”.
He said SWW had initially denied the issue was anything to do with the company, and even advised people the tap water was safe to drink on Tuesday.
The following day, SWW bosses reversed their advice and urged people to boil their water before using it.
Mr Mangnall said: “The predominant failure is the fact that earlier in the week, South West Water was asked whether or not this was to do with their network and they categorically ruled it out, only for them to change their position 24 hours later, which I think is contemptible and just generally incompetent – and it’s put a lot of people’s health at risk.
“That, to me, is one of the most serious indictments because they were made aware of this by a large number of people, including myself who raised this with them, and so to not actually respond in a manner that would safeguard public health, I think is deeply problematic.”