Security systems will alert police to attempts to tamper with supplies
New security is being introduced to protect Devon and Cornwall's water supplies from sabotage.
The work follows a visit to the area by MI5 officers, to study vulnerabilities in Britain's water infrastructure.
Senior police sources say the security measures are not in response to a specific threat, but precautionary.
The work involves systems to alert the authorities about attempts to tamper with supplies and prompt the police to send armed officers.
The cost of the new security is being met by South West Water and the government.
Bill Tupman, an expert on counter-terrorism from Exeter University, told BBC News: "The problem is trying to predict the mind of the al-Qaeda planner; there are so many things they might do.
"And it is also necessary to reassure the public that we are trying to outguess the al-Qaeda planner and we are in the process of protecting them from any threat."
South West Water declined to be interviewed, but said in a statement that it was not company policy to discuss security matters.
The Home Office said it did not comment on the work of the security services.