It'll be another year before the country's sickest children can receive treatment at a new £150m-state-of-the-art hospital in Edinburgh, as ventilation problems in its critical care unit force further delay to its already long delayed opening.

The mistake dates back to 2012, when NHS Lothian made an error in the tender documents.

This led to inadequate ventilation in an area where contamination could be fatal for patients, and will add another £16m to the tab, as workers optimistically work toward an autumn 2020 opening.

It's a disappointing setback to families who have been anxiously waiting to move their children into the 242-bed Royal Hospital for Children and Young People, which was supposed to open in July.

The repair work isn't the only extra cost - it's estimated the health authority must pay £1.4m a month to the owners of the site to take care of things such as security, lighting and heat.

Health secretary Jeane Freeman said she sympathised with families and staff and said the health board's mission to plan and build the facility "has not been delivered."

She said: "I am of course bitterly disappointed that a mistake made in 2012 was not picked up earlier.

"This is a publicly funded project of strategic importance, which has not been delivered by NHS Lothian in compliance with the standards and guidance.

"The delay we now face will be borne by NHS Lothian staff, by patients and their families and the additional cost will be to the public purse."

Freeman said a report by NHS National Services Scotland performed water, ventilation and drainage checks across the hospital, located in Little France, and didn't find any "widespread contamination."

Freeman insisted while the work would be carried out as quickly as possible, the hospital would not open until all remedial actions had been "completed, tested and found to be fully compliant".

The department for clinical neurosciences - in a different part of the hospital could open in the spring.

As well as calling for action on ventilation and water, the report also recommended continued monitoring of the drainage and plumbing systems, though it says these areas are "low risk."

Opposition MSPs have been calling for a public inquiry into the delays, while First Minister Nicola Sturgeon announced this month a new watchdog to oversee NHS building projects.

Shadow health secretary Miles Briggs said: "This confirms that patients and their families will have had to wait nearly a decade longer than promised for this much-needed hospital ... all over the world new hospitals are built and delivered on time and on budget, yet under the SNP, from the very outset, this project has been a farce."

Scottish Labour health spokeswoman Monica Lennon said: "The delays to the opening of the new children's hospital have been a disaster, but we are still no clearer on where the responsibility lies."