Almost 40 years after the Hillsborough stadium disaster, which claimed the lives of 97 people, the families of those killed or injured reached a milestone last year in their long struggle for justice.

It came in the form of the Hillsborough law, which would compel public bodies and officials, such as the police, NHS and local authorities, to assist investigations with full and truthful information or face criminal sanctions.

In practice, it would mean that those who lie, evade or withhold information would face prosecution, potentially resulting in prison sentences.

But the Hillsborough law has hit a significant roadblock. Here we explain what has happened and what may happen next.

Why is the Hillsborough law so significant?

This legislation – officially called the public authority (accountability) bill – seeks to address concerns that public bodies have been able to withhold information or mislead investigations in which they face criticism.

The most high-profile example of this is the Hillsborough disaster, in which a crush during an FA Cup semi-final on 15 April 1989 led to the deaths of 97 Liverpool fans. More than 100 witness statements given by police officers were found to have been amended to remove or change comments unfavourable to South Yorkshire police, who were responsible for fan safety on the day.

In 2016, nearly 30 years after the disaster, a landmark inquest found that those who died were unlawfully killed – and that the behaviour of Liverpool fans played no part in the tragedy, as officers had claimed.

For Keir Starmer, the Hillsborough law is one of his government’s flagship policies. He placed it at the heart of his speech to Labour party conference in September, where he was introduced on stage by Margaret Aspinall, whose 18-year-old son James died in the disaster. With this new law, he said, “injustice has no place to hide”.

What is happening now?

Last week, Labour introduced a new amendment to the bill that would bring intelligence agencies such as MI5 and MI6 into its scope – subject to the approval of the chief of those services.

In practice, this would mean spies would be compelled to give full, honest and transparent evidence to inquiries regardless of whether it would be unfavourable to the agency.

However, campaigners said this allowed intelligence officials to decide what information is released to investigators, potentially allowing cover-ups to continue. It prompted a fierce backlash from many Labour MPs, mayors and families of the victims.

The government pulled this amendment late on Sunday, just 24 hours before it was due to be laid in parliament, in the wake of the criticism.

Why is this so damaging for Labour?

Starmer has wedded himself to the Hillsborough law – it was one of the key pledges in Labour’s 2024 manifesto – so failing to introduce it in full would be hugely politically damaging.

Paula Barker, the Labour MP for Liverpool Wavertree, said the government’s insistence on the amendment would feel like the “ultimate betrayal” to the people of Liverpool. Other Labour MPs said a watered down bill was “untenable and unacceptable”.

Charlotte Hennessy, whose father, James, died in the tragedy, said the bill was “95% there” but urged the spy clause to be dropped, telling Starmer: “Please don’t destroy all that we have achieved.”

Labour has been accused of backtracking or U-turning on several policies since it was elected last June. But failing to pass a full Hillsborough law would be one of the most politically toxic.

What happens next?

The Hillsborough law is due to enter its penultimate stage in the House of Commons on Monday before going to the House of Lords.

Ministers said on Sunday they would meet with campaigners and MPs to discuss any planned changes that may be introduced when it reaches the Lords.

This, then, is a critical stage. MPs and campaigners have vowed not to support any “weakened” bill and losing their backing could delay the law being introduced and be hugely damaging for the prime minister.

The campaign group Hillsborough Law Now said on Monday that the pause was “frustrating” but “gives the chance for government to listen further”. It added: “The Hillsborough law must leave the Commons with the families’ support.”