Tests for bacteria in the water supply at the Royal Hospital for Children have been extended to the neighbouring Queen Elizabeth University Hospital.
Four children are now receiving treatment for infections which may be linked to the water supply at the RHC in Glasgow.
NHS Greater Glasgow and Clyde (NHSGGC) has confirmed testing has been extended to four wards at the QEUH.
Infection control measures are also in place for patients with low immunity.
Health Secretary Shona Robison has apologised to patients and families affected.
In a statement to MSPs at Holyrood on Tuesday afternoon, she said "appropriate precautionary measures" were being taken by NHSGGC to protect patients.
NHSGGC said it is working with experts from Health Protection Scotland, Health Facilities Scotland and Scottish Water.
The health board revealed the probe had been extended after a source told BBC Scotland that taps and showers had been taped up in QEUH wards and only bottled water was allowed.
On Friday it emerged three children were receiving treatment at the RHC but a fourth has now shown symptoms.
An NHSGGC spokesman said: "These bacteria can pose a risk to patients whose immunity is compromised, however we have put in place robust infection control measures to protect our patients.
"We have now extended this testing and infection control measures to four wards treating the most immunity compromised patients in the Queen Elizabeth University Hospital to ensure that we take every precaution."
The board also confirmed the children who became ill last week are still recovering.
The spokesman added: "Three children continue to receive treatments for infections that may be linked to bacteria found in the water supply.
"Tests are ongoing to confirm if they are indeed linked.
"A fourth patient has shown symptoms and has been readmitted to the RHC.
"There are no reports of any patients being infected by bacteria from water in the adult wards."
A series of infection control measures were introduced to the affected RHC wards at the weekend and water filters are to be installed.
'Worry and concern'
The statement concluded: "As a result, it is hoped that the full water supply will return to normal within 48 hours after appropriate testing has been carried out and it is anticipated this will be mirrored at the QEUH.
"We would like to thank the patients and families in the wards affected in the RHC for their continuing patience and support while the testing was carried out and fitting of filters is being undertaken."
During the health secretary's statement to the Scottish Parliament Labour MSP Anas Sarwar said parents had told him the problem had been ongoing for almost three weeks.
He said it had resulted in the inability of young cancer patients to bathe properly, with some taking taxis to other sites to use facilities, he said.
He added: "The news of contamination of the water supply in the cancer ward at the children's hospital in Glasgow has caused understandable worry and concern for parents of very sick children."
Ms Robison said she had first been made aware of the matter on 11 March, with Scottish government officials informed prior to that.
"I absolutely understand the worry and concern of parents," she said.
"I have been assured by the health board that they have indeed been keeping parents informed.
"Of course I would apologise to the parents and the children for the inconvenience that they have experienced, but I am sure everyone would understand the most important thing here is safety, and if the shower heads and taps are being tested and investigated then that has to take its course."