The source of lead in drinking water that has delayed the opening of Perth’s new $1.2 billion Children’s Hospital is corroded brass fittings.

More than 1200 assembly boxes located near water outlets containing brass valves will have to be replaced by stainless steel versions, a report by the chief health officer recommended.

The hospital has become a political scandal due to a two-year delay so far and cost blowouts largely blamed on contractor John Holland, and the latest news adds doubt as to when it will open next year.

“It is with great disappointment, given the work that now needs to be undertaken, the hospital will not open this year, however, we are on a clear path forward to rectify this issue,” Health Minister Roger Cook said.

John Holland is yet to comment, with a spokeswoman saying it was still assessing the report.