Eau de Paris, the capital's state-run water company, has banned access to six sensitive sites to all but key personnel following the Government's decision to declare a state of emergency for three months.
"Our eight security agents are the only ones to be accredited by the Defence Ministry and are in permanent contact with the terrorism cell of Paris police headquarters," a spokesman told Le Parisien.
The storage and treatment sites have protective fencing and special sensors to detect intrusion. To limit the risk of contamination, Eau de Paris has increased the amount of chlorine added to water at five supply sites.
"The dose injected has been raised," said Celia Blauel, president of Eau de Paris. Chlorine is a key indicator of any anomaly in the water. "When the chlorine level drops, it means there is biological contamination," she said.
It is tested all along the supply chain, meaning that water authorities can "cut off supply in a contaminated area if necessary without stopping the entire system," she said.
Eau de Paris produces a million cubic metres of drinking water per day but the capital only consumes half of that amount. "So we always have a day's lag in terms of consumption," said Blauel, describing the system as "extremely safe".
The heightened security came amid reports that a dozen protective suits against highly contaminative viruses are missing from Paris' Necker hospital.
Three times as many protective boots made of polyethylene - resistant to chemical agents - have also vanished, along with gloves and anti-bacterial masks.
The hospital said: "The disappearance of this limited amount of equipment was noticed on [Thursday] and a complaint has been filed." Philippe Goujon, Mayor of Paris' 15th district, where Necker is located, said the theft was "naturally worrying".
It emerged last week that hospitals and emergency services across France have been supplied with the most powerful antidote to sarin and other nerve gas chemicals for the first time.
Under a November 14 decree, the army's medical service was ordered to distribute stocks of the drug atropine.
Prime Minister Manuel Valls said that the country could rule nothing out when it came to terrorists bent on mass murder, including "chemical and biological weapons".
Iraqi and American intelligence have warned that Isis is aggressively pursuing development of chemical weapons, setting up a branch dedicated to research and experiments with the help of scientists from Iraq, Syria and elsewhere in the region.
French intelligence agents are convinced that Isis is producing mustard gas, after obtaining samples from in Syria. However, US intelligence officials do not believe Isis has the capability to develop sophisticated weapons such as nerve gas.
With Paris trying to return to normal life but with all public gatherings banned until November 30, terror fears spread to Brussels after the Government warned of a "precise" and "imminent" threat of a Paris-style attack on the Belgian capital.
Heavily-armed soldiers and police officers patrolled streets across Brussels as the metro system was closed and officials told people to avoid crowded areas such as shopping centres.
The Belgian terror alert came as the massive manhunt continued for Salah Abdeslam, the only known alleged Paris attacker to have escaped alive.
He is known to have returned to Belgium after the attacks, possibly after pulling out of a planned suicide bombing at the last minute.
French authorities have said the attacks were planned in Brussels by Abdelhamid Abaaoud, 28, who fought for Isis in Syria and was killed in the siege of an apartment in the Paris suburb Saint-Denis last week. His fingerprints were found on one of the Kalashnikovs used to gun down people at bars and restaurants.
Salah Abdeslam is said by officials to have known Abaaoud in jail.
Fears that Abdeslam, whose elder brother Brahim blew himself up at a cafe in Paris, or his associates may still pose a security threat led Belgian authorities to cancel a football match between Belgium and Spain.
The Belgian authorities have charged three people with involvement in the attacks, which Isis said it carried out. A Moroccan national was reportedly charged with the murder of 130 people for his alleged part in the planning. The suspect named A. Lazez is thought to have helped Salah Abdeslam when he returned to Belgium.
Police are trying to determine the meaning of an SMS message Lazez received on his phone while in custody. It read "The Jew is not there". Two others are being held in Brussels. Mohamed Amri, 27, and Hamza Attou, 21, went to fetch Abdeslam from Paris but deny any knowledge of his involvement in the attacks.
Turkey detained Belgian citizen of Moroccan origin Ahmet Dahmani, 26, on suspicion of links to the Paris attacks.
Telegraph Group Ltd
