Anas Sarwar has asked why the death of a child linked to infections at the Queen Elizabeth University Hospital (QEUH) in Glasgow was not reported for investigation.
The Scottish Labour leader raised the issue at Holyrood during FMQs on Thursday.
It comes as an inquiry set up to investigate construction of the hospital continues to hear evidence.
Last month, it emerged the Crown Office and Procurator Fiscal Service (COPFS) instructed Police Scotland to investigate four deaths at the flagship hospital – three of those concerning children and the fourth concerning a 73-year-old woman.
Sarwar this week passed on details of the death of a second child at the hospital to the Procurator Fiscal.
At the Scottish Parliament, he also hit out at the Greater Glasgow and Clyde health board as he said the families had had to fight to uncover the truth.
“Two years ago, I stood in this chamber and revealed what brave NHS whistle-blowers had uncovered about water contamination at the Queen Elizabeth University Hospital in Glasgow,” he told First Minister Nicola Sturgeon.
“It was met with denial, delay and attempts to bully and silence by the health board.
“Two years on, we’ve had a discredited independent review, a case note review, the commencement of a public inquiry, and ongoing police invesitgations.
“Every step of the way, we have had to fight the system to bit by bit, piece by piece, uncover the truth.”
He continued: “Thanks to the case note review, we know that two children’s deaths were linked to hospital infections.
“There is now a criminal investigation into one of those deaths, Milly Main.
“But the health board only referred Milly’s case to the Crown when Milly’s family applied for a Fatal Accident Inquiry (FAI).
“They didn’t take the opportunity then to refer the second child and they didn’t take the opportunity when the case note review was published.
“I met with the Crown, they didn’t know the details of this second child and had to ask me to provide those details, again thanks to the bravery of whistle-blowers.
“That case note review was commissioned by the Scottish Government. Why wasn’t that child’s death reported for investigation?”
The First Minister rejected suggestions that the independent review had been “discredited” as she outlined the seriousness with which she and the Scottish Government takes the issue.
She responded: “These are really serious matters that the Government has and continues to take seriously.
“The Government commissioned the independent review. I don’t accept that it is a discredited independent review, but the Government also accepted that there had to be further process in order to ensure that parents and families affected by what had happened at the Queen Elizabeth University Hospital was fully and properly investigated.
“So, this government established the public inquiry which is now underway and that inquiry will take its course and is completely independent of government and indeed of course of Greater Glasgow and Clyde health board.”
Sturgeon continued: “As Anas Sarwar has also said, there is a live police investigation into some of the cases that have been discussed in this chamber before.
“For all of these reasons, it would not be right, appropriate, or indeed helpful to the families concerned for me to get further into the detail of any of these cases given the processes, the independent processes, that are underway.
“But, I want to leave no-one, as I have done before, in any doubt about how seriously I take these issues, how seriously the Government takes these issues, and how determined we are, through the processes we have established in the form of the public inquiry, to get to the answers, to get to the truth and then all of us as a parliament of course will have the opportunity to reflect on those findings and consider what further action is necessary,”