The force said 69 cases may be affected by the “unexplained reduction in data stored on its systems”, as it can no longer access the footage which was recorded by officers between July 2020 and May 2023.

A spokeswoman said it appears to have been deleted and “there is no suggestion our systems have been hacked”.

Assistant Chief Constable Rick Alton said: “We’ve brought in digital forensics specialists to explore how this data loss occurred and to establish if the data can be recovered.

“We will provide an update concerning this when further work has been undertaken. Whilst we remain hopeful, there are no guarantees.”

He added: “I am deeply sorry this has occurred and after we became aware of the loss in data, we notified the Information Commissioner's Office and began work to attempt to recover it.

“We’ve had a team of people working around the clock to manually check each case currently within the criminal justice system.”

The force said data includes footage which was filmed by officers when they attended incidents and it can be used as evidence in criminal and civil cases.

Dr Alan Billings, South Yorkshire Police and Crime Commissioner, said the breach is “disappointing and concerning” and the force “need to understand quickly what has happened and why”.

“There may be implications for victims and witnesses and the wider criminal justice system as some of this footage may be evidence in upcoming court cases,” he said.

“The force is working through the implications and direct contact is being made with those affected.”

He added: “I have been briefed by the Deputy Chief Constable and will continue to receive updates as the work continues to recover the data.

“I will also be seeking a full explanation as to how this incident occurred and what measures will be put in place to prevent it happening again.”

It comes as Police Service of Northern Ireland (PSNI) is dealing with a major data breach, after the personal details of more than 10,000 officers and staff were mistakenly published online.

Surnames and initials of 10,000 police employees were accidentally included in a freedom of information response.

PSNI Chief Constable Simon Byrne said he believed the information was in the hands of dissident republicans.

Many officers have expressed concern for their safety in Northern Ireland, where police are under threat from terrorists – with the current level of threat assessed as severe, meaning an attack is highly likely.

On Monday, Christopher Paul O’Kane, 50, appeared in court in Co Antrim, charged with two terror offences relating to the data breach.

The court was told that he was arrested at his home on Friday and two phones were discovered in his bedroom.

A PSNI officer told the court that O’Kane was in possession of a spreadsheet containing names from the data leak on one phone and sections were highlighted.

He was charged with possessing documents or records likely to be useful to terrorists and possession of two mobile phones for use in terrorism.

South Yorkshire Police says footage which could be used as evidence in dozens of cases has been lost