The police criminal inquiry into the Post Office has identified dozens of persons of interest so far, as a team bolstered to 100 officers investigates the actions of executives, legal teams and civil servants connected to the Horizon IT scandal.

The investigation, which the police describe as unprecedented in size and scale, is in the first instance examining potential offences of perjury and perverting the course of justice by those involved in making “key decisions” on Post Office investigations and supporting prosecutions of branch owner-operators.

However, a second phase, which is being developed concurrently, is looking at “wider offences” and decision-makers involved more broadly at the Post Office, as well as at Fujitsu, which developed the controversial Horizon accounting software.

Stephen Clayman, the Met commander overseeing the police investigation, said officers were “looking at the actions of prominent individuals” beyond those directly involved in making decisions on Post Office investigations and supporting prosecutions.

“We will go where the evidence takes us,” Clayman said. “We are looking at the Post Office and Fujitsu and anything wider. We will cast the net wider in terms of culpability.”

Michael Norman, the senior investigating officer, added that police were also looking at “investigators, solicitors, barristers and people within Fujitsu as well”.

“As others [persons of interest] come into scope we will look at those as well, if they become raised to suspect status,” Norman said. “It is very fluid. The issue of corporate liability, corporate culpability, is always open.”

Norman said that to date the police had interviewed three individuals under caution, dating back to 2021, with the most recent in September this year.

Clayman said prosecutions would not reach trial until 2027, in part due to the “unprecedented” scale of the investigation, which is reviewing more than 1.5m documents.

The police will await the publication of Sir Wyn Williams’ two-year public inquiry into the Post Office and the Horizon IT scandal before moving forward to charging.

Clayman said: “No key decisions will be made around submissions and charging decisions until the final report is delivered and thoroughly reviewed by the investigation team and the Crown Prosecution Service. We are looking at 2027 [for trials] realistically.”

He said the team of police officers and staff across the country had been bolstered to 100, up from the 80 involved back in May.

“The scale of the task ahead is unprecedented,” Clayman said. “I do know that if you take into account Post Office criminal and private prosecutions, civil claims and contract withdrawals, there are potentially thousands of victims who we are working hard to identify.”

More than 900 post office operators were prosecuted between 1999 and 2015 because of faulty Horizon accounting software that made it look as though they had been committing fraud.

The police investigation, which was started in 2020, has launched a public portal to allow anyone to submit information and documents.

Clayman said: “We have been building a larger investigation team made up of officers across all forces. All forces are contributing to the build of a national team. This is a truly national operation in scale and should be resourced as such.”