The number of people at risk from the cryptosporidium bug in the greater Belfast and Lisburn areas has doubled in the last 24 hours to more than 100,000, leading the Department of Regional Development to describe the outbreak as a "major incident".
The Department yesterday said an internal investigation had confirmed that the bug - a type of parasite which can cause severe diarrhoea, stomach cramps and nausea - was "most likely" carried in the water supply, although transmission was also possible by touching animals or lack of personal hygiene.
A warning issued by the Department to people in the Poleglass area at the outskirts of west Belfast to boil all drinking water was yesterday extended to the Dunmurry and Finaghy areas of south Belfast and to Lisburn town, Co Antrim, increasing the number of households affected by the outbreak to 28,000.
Health officials confirmed 61 cases of the infection had been diagnosed in the past 10 days, but it is believed the total number may already be in the hundreds.
The North's First Minister, Mr David Trimble, expressed "deep concern" at the spread of the outbreak and called for an urgent report from the Department of Regional Development detailing the situation, as well as measures taken by the Water Service to deal with the problem.
An SDLP MLA for south Belfast, Dr Alasdair McDonnell, who is a local GP, criticised the Water Service for not reacting more quickly, describing its performance as "sheer incompetence".
"This situation has not just arisen because of a lack of funding, but also as a result of incompetence on the part of the Water Agency in providing effective detection procedures," Dr McDonnell added.
"The community as a whole deserves the right to clean and healthy drinking water. This is not the situation in many parts of Northern Ireland at present."
His party colleague and fellow GP, Dr Joe Hendron, said the confirmed cases were only the "tip of the iceberg". His surgery had been treating a "fair number" of patients affected by diarrhoea and stomach cramps, he added.
"As the chairman of the Assembly's health committee, I will be raising some serious questions next week. I want to know exactly what the situation is and how large the affected area is likely to be - after all, we have no indication that it will stop at the currently defined area."
Earlier, the Minister for Regional Development, Mr Gregory Campbell, released a statement blaming the North's antiquated water system for the outbreak.
Between £2.5 and £3 billion sterling would be needed to upgrade the 110-year-old system to an acceptable standard, the Minister added.
People in the affected areas yesterday complained they had received little information from the Water Service. Local shops reported that all supplies of bottled water were sold out by the afternoon, and pub landlords said they were not using ice cubes.