The new £150m hospital for children in Edinburgh may have to be "ripped down" amid safety concerns, a senior trade union official has warned.
Tom Waterson said drainage at the building is a more pressing matter than the ventilation issues which saw the postponement of its official opening.
He also claimed NHS Lothian had paid millions of pounds in a settlement to help resolve some of the problems.
The health board said reviews into the delay are already under way.
As well as focusing on ventilation they will also look at drainage and water systems as a priority.
The development comes after BBC Scotland revealed NHS Lothian is paying millions of pounds to a private consortium for a hospital it cannot use.
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".
But despite that the contractors continued to press ahead with construction.
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 said he is also speaking as a parent.
He 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."
The new Royal Hospital for Children and Young People, which will have over 200 beds, was supposed to open in July, but health secretary Jeane Freeman overruled NHS Lothian after last-minute inspections found safety concerns over its ventilation system.
The project has been marred by disputes between the local NHS board and IHSL Lothian, the consortium responsible for building the hospital.
Although the investigation is expected to be finished this year, no date has been offered by the Government or NHS Lothian for when the hospital will open.
Mr Waterson, who is the chair of Unison's Scottish health committee, represents 12,500 members in NHS Lothian, including staff who were expected to switch from the existing facility in Edinburgh.
A Scottish government spokeswoman told The Herald: "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."
The spokeswoman confirmed Ms Freeman will meet again with NHS Lothian staff representatives in the coming weeks.
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.
"The reviews and subsequent reports will be provided to the cabinet secretary for health and NHS Lothian."