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Met Commander Stephen Clayman said: “The teams need to be really meticulous and [pay] attention to detail, but actually we are making some real progress.”

The police investigation has so far identified seven suspects and is reviewing 45 to 50 others. However, no files will be passed to the Crown Prosecution Service until after the final report from the public inquiry, expected later this year.

Former sub-postmaster Tim Brentnall said victims were “desperate to see some kind of accountability”, but emphasised that police should “do it properly”.

The Horizon IT system, introduced in 1999, incorrectly recorded financial shortfalls in Post Office branches, leading to the wrongful prosecution of more than 900 sub-postmasters – some of whom were imprisoned. A number of those affected died before seeing their names cleared.

Legislation to quash the convictions en masse was passed last year.

The police inquiry, Operation Olympos, began in 2020. Two suspects were interviewed the following year, and after the inquiry’s public hearings concluded in December, another two individuals – both men in their sixties – were interviewed under caution, meaning their statements could be used in court.

Commander Clayman said investigators had initially focused on “those at the front line – the Post Office investigators, solicitors, those who were involved in the immediate decision-making”.

He added: “We are beginning to scope looking at wider management. That will happen, and is happening, it will just take time to get there.”

While he remains confident that criminal trials will take place, Commander Clayman acknowledged that proceedings may not begin until 2028. The chair of the statutory inquiry, Sir Wyn Williams, is due to publish the first volume of his final report on 8 July, with a second volume expected later in the year.

Only once both parts are published will the police begin a detailed review, submit files to prosecutors, and await court dates.

“This isn’t uncommon,” Commander Clayman said. “Other large investigations linked to a public inquiry have exactly the same thing. And I really do understand the frustration for those who are at the centre of this, who are the victims.”

A Post Office spokesperson said: “The Post Office has co-operated fully and openly with the Metropolitan Police since early 2020 to provide whatever information it needs for its investigations, and we continue to do so.”