Ministers have announced the inquiry into the serial killer Lucy Letby’s crimes will become statutory after pressure from bereaved families and their lawyers.

A non-statutory inquiry had previously been declared to be the “most appropriate option” to look into the circumstances at Countess of Chester hospital. However, the health secretary, Steve Barclay, announced on Wednesday it would have statutory powers.

But what is a statutory inquiry? Here we look at what it will entail and how its terms will differ to those of a non-statutory inquiry.

How do statutory inquiries work?

Statutory public inquiries are established by a government minister, but, once set up, work independently of Westminster. They run according to the rules in the Inquiries Act 2005 and Inquiry Rules 2006 and have the advantage of having legal powers to compel witnesses to give evidence, provide legal safeguards, and can set limits upon the government’s control of the inquiry.

Inquiries always have a chair, often a judge, appointed by the minister. Any person or organisation can be made to produce relevant documents and it is a criminal offence to intentionally withhold a document required by the inquiry or deliberately to obstruct its work.

Inquiries are public and in general the chair must take reasonable steps to ensure the public can watch hearings and view the record of evidence. They must produce a written report with their findings and recommendations and this, too, must be made public.

How does a non-statutory inquiry differ?

Non-statutory inquiries are not required to meet the requirements of the Inquiries Act and can therefore operate with greater flexibility than statutory inquiries. While this may allow them to progress at a quicker pace, the discretion afforded to the chairs of non-statutory inquiries can mean they are not as publicly accountable.

Non-statutory inquiries also lack the power to compel witnesses to attend or give evidence under oath – with a reliance instead on the relevant parties choosing to cooperate with these inquiries.

In June, an inquiry into 2,000 mental health deaths in Essex NHS trusts was given statutory status after pressure from bereaved relatives and the then chair of the inquiry, Dr Geraldine Strathdee.

What other statutory public inquiries have there been?

The UK Covid-19 inquiry began last year and was set up to examine the UK’s response to and impact of the pandemic.

During her time as home secretary, Priti Patel established two of the most recent public inquiries at the end of 2019; the Manchester Arena inquiry into the 2017 terror attack and another into the mistreatment of individuals who were detained at Brook House immigration removal centre.

The Grenfell inquiry, overseen by Sir Martin Moore-Bick, was formally set up in 2017.

The independent inquiry into child sexual abuse was launched in 2014 and published its final report in October 2022

Why do inquiries take so long?

Statutory inquiries usually take years to complete and can be delayed by criminal investigations and staff turnover. Concurrent criminal investigations and trials will often demand a similar list of witnesses and experts.

Inquiries must review thousands of documents and take witness testimony. Even under ideal circumstances they take time due to the sheer volume of evidence they need to consider.