The families of two people who took their own lives in a young offenders institution have urged Scotland’s First Minister to take action as a fatal accident inquiry (FAI) begins into their deaths.

Katie Allan, 21, and William Lindsay, 16, were found dead in their cells in separate incidents at Polmont Young Offenders Institution in 2018, and their deaths will be investigated at the inquiry starting at Falkirk Sheriff Court on Monday.

In a statement outside court, the solicitor representing the families, Aamer Anwar, said both families want Humza Yousaf to work to remove Crown immunity, which means the Scottish Prison Service (SPS) is exempt from prosecution for the deaths; set up an independent body to investigate prison deaths; and impose a year’s limit for the start of an FAI.

He said they also want the First Minister to hold SPS to account and ensure families get automatic legal representation at an FAI.

He said: “Whilst the families recognise today as a milestone in their journey for the truth, it is certainly not the end of the process.

“The First Minister, Humza Yousaf, must not betray the many promises he made to these families whilst Justice Minister, he cannot escape behind a veil of silence because there is now an FAI.”

Ms Allan’s parents, Linda and Stuart, stood with Mr Aamer as he made a statement on their behalf that they “believe that the SPS have no accountability for the avoidable deaths of young people”, saying that while the Crown Office found “evidence to pursue a criminal prosecution against the Scottish Prison Service but Crown immunity prevents them from doing so”.

He added: “They feel they have been failed by the Scottish criminal justice system since the day of their daughter’s death yet, today, are once again facing another process which cannot apportion blame.”

Mr and Mrs Allan are expected to give evidence during the early days of the FAI, which is due to last three weeks, while Mr Lindsay’s brother, John Reilly, will represent his family.

In a statement on behalf of Mr Reilly, Mr Anwar said Mr Lindsay’s death caused “unspeakable grief”, adding: “John hopes that sufficient change and safeguards are put in place, to ensure that no family need never go through what his family has since the day William entered HMP YOI Polmont.”

The inquiry will explore the circumstances of both deaths, with particular focus on the SPS’s “Talk To Me” strategy in relation to the prevention of suicide in jails.

It will seek to establish what, if any, precautions could have been taken, or could be implemented to minimise the risk of future deaths in similar circumstances.

Mental health nurses are among those expected to give evidence.

Mr Lindsay, who had been in care repeatedly, died at Polmont on October 7 2018 – three days after being admitted as there was no space in a children’s secure unit, despite him having a history of making attempts on his life. He had been sent there on remand.

His mother and two sisters have died during the five-year wait for an inquiry.

Ms Allan, a student at Glasgow University, was found dead on June 4, 2018 while she served a 16-month sentence for drink-driving and causing serious injury by dangerous driving, which she had admitted.

A SPS spokesperson said: “The loss of any person in our care is devastating and we recognise the profound impact it has. Everyone who enters custody is assessed to identify what support is needed.

“Given a fatal accident inquiry is to be held imminently into the deaths of Katie Allan and William Lindsay, it would be inappropriate for us to comment further.”

A spokesperson for the Crown Office said: “We acknowledge that the wait for these proceedings has been too long and has caused distress for Katie and William’s families.

“The FAI will allow a public airing of all the evidence at which families and interested parties will be represented.

“The families and legal representatives will continue to be kept updated as the inquiry progresses.”

A Scottish Government spokesperson said: “Our thoughts are with every family who has been bereaved by suicide in prisons.

“We cannot comment on these cases with the approaching fatal accident inquiry and we will carefully consider the outcome.”