The advice was issued by public health officials after 32 people became unwell due to a water-borne micro-organism, cryptosporidium, which causes serious and prolonged stomach upset.
Three Valleys Water, which serves Hertfordshire and north-west London, broadcast warnings to 300,000 families after samples of water were found to be carrying the germ.
Christine Murphy, of the Public Health Service Laboratory, said: "The number of people affected by this latest outbreak is fluctuating.
"We are advising worried families to boil their water for the next three days.
"We always expect to see between 4,000 to 5,000 cases each year but each outbreak is taken seriously."
Cryptosporidium causes diarrhoea which can last up to three weeks and is potentially fatal for children or people with deficient immune systems.
The organism, which belongs to a group of protozoa and occurs naturally in farm animals, can be washed into the water supply from nearby agricultural land. It is usually spread through contaminated water or contact with an infected person, but unpasteurised milk and offal also carry risks.
A spokesman for Three Valleys Water said: "We are advising customers to boil their water as a precautionary measure and there will be a full investigation into this outbreak.
"Although there is no specific treatment for the illness, we are also advising sufferers to drink plenty of fluids to cleanse their system."
According to environmental specialists this latest outbreak is unusual because the water is from a ground source supply.
Philip Lightowlers, of the Environmental Data Service Journal, said: "We usually see crypto in surface water supplies which have the most rigorous treatment checks.
"There is a possibility that this ground water, which is naturally filtered through rocks, didn't undergo the same stringent tests, but we will have to wait for the investigation to see whether this is the case."
A spokesman for West Hertfordshire Health Authority said the outbreak had not reached epidemic levels. "But we are seeing about three times as many cases as we would normally expect this time of year.
"At the moment, no one is seriously ill or in hospital to our knowledge, but we are monitoring their progress closely."
In 1989 an outbreak of cryptosporidiosis in Humberside prompted a Department of the Environment recommendation for water companies to keep a close watch on heavy rainfalls and dumping. The report also suggested the privatised utilities should monitor water treatment regularly.
Despite increased water observation, there have been five serious outbreaks since 1992.
Catherine Comben, of the Drinking Water Inspectorate, said: "We will be carrying out a full investigation into this incident to see if the company could have done anything to prevent this latest outbreak."