A senior trade union official has claimed that the new £150m Sick Kids hospital in Edinburgh may have to be 'ripped down'.
Tom Waterson, chairman of Unison Scotland's health committee, said drainage at the building is the most pressing matter facing the troubled project.
He also claimed NHS Lothian had already spent millions of pounds to help resolve some of the problems.
The health board said reviews into the delay are already under way.
NHS Lothian has been haemorrhaging millions of public money for the delayed Sick Kids hospital in Edinburgh, which is yet to open to patients.
Health chiefs are paying out millions to private consortium IHSL which built the new state-of-the-art hospital, despite the opening being cancelled at the 11th-hour last month due to safety fears over its ventilation system.
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Mr Waterson said: "I've been speaking to senior staff within NHS Lothian over the last two or three weeks and more and more have been coming to speak to me.
"They are telling us that they have concerns primarily over drainage at the site. People are unable to confirm whether the drainage that has been put in, is in fact fit for purpose.
"We need to find out what's happening before everyone moves in. We can't wait to find out later on and just keep our fingers crossed."
Mr Waterson said the problems date back 18 to 36 months when senior staff were "alerted to shortcomings in the drainage".
He added: "There is a school of thought that they might have to rip it down.
"How do you fix drainage in a building when it's "x" number of feet beneath the building? I'm not an engineer, but it's not going to be easy.
"I'm extremely worried."
The father-of-two added: "My daughter had to use the Sick Kids last year and the service was excellent.
"My concern is that we can't use the new building until we know that it's 100% safe.
"The Scottish government had three people on the project board from day one. They would have been fully aware of this issue 36 months ago."
A Scottish government spokeswoman said: "The health secretary has made clear that her greatest responsibility is the safety of patients, and for this reason decided to delay the move of patients, staff and services to the new hospital.
"Patients and carers have been contacted directly to confirm appointment arrangements and a dedicated helpline remains in place.
"She recognises that many staff share her frustration following the announcement of the delay."
Prof Alex McMahon, nurse director at NHS Lothian, said: "There are a number of independent reviews and investigations underway to verify and provide assurance that all aspects of the Royal Hospital for Children and Young People, the Department of Clinical Neurosciences, and Child and Adolescent Mental Health Services meet the appropriate standards before it becomes operational.
"The decision to delay the move followed the identification of a problem with ventilation in critical care. Given the pause in occupation, the commissioned reviews will focus on ventilation and will also look at drainage and water systems as a priority.
"An Oversight Board, made up of Scottish government, NHS Lothian, National Services Scotland and Scottish Futures Trust, has been established in order to provide co-ordinated advice on the readiness of the hospital to open and on the migration of services to the new facility."