The police criminal inquiry into the Post Office Horizon IT scandal faces a five-year delay unless it is handed millions in extra funding and nearly 100 more staff, according to the chief officer in charge.

The Metropolitan police commander Stephen Clayman said he needed to nearly double the number of investigators to 210 to meet a deadline of late next year or early 2028 for submitting files to prosecutors.

The Home Office recently gave a special grant of £2.8m to the investigation but Clayman said its projected budget was up to £19.3m, leaving a £16.5m shortfall.

More than 900 post office operators were prosecuted by the Post Office between 1999 and 2015 because of the faulty Horizon accounting software from the Japanese technology company Fujitsu that made it look as if they had committed fraud.

The scandal has been described as the worst miscarriage of justice in British history, and was the subject of the acclaimed ITV drama Mr Bates vs the Post Office, which aired in January 2024. Ministers introduced legislation later that year to exonerate people who had been wrongly prosecuted.

The police investigation has previously been described as unprecedented in size and scale, and is the first to examine potential offences of perjury and perverting the course of justice by those who made key decisions on Post Office investigations.

Police will await the full publication of the conclusions of Sir Wyn Williams’s two-year public inquiry into the Post Office and the Horizon IT scandal before moving to charging.

Part one of the inquiry’s findings, which focused on the human impact and on financial redress, was published last year. No date has yet been set for the release of the second part, which is expected to focus on the Horizon system’s flaws, the culture within the Post Office and Fujitsu and how post office operators came to be wrongfully prosecuted.

Clayman said the police investigation, codenamed Operation Olympos, is “hugely complex” and detectives already hold 8m documents. “This number is set to grow, with many of these documents needing to be forensically reviewed and considered,” he said.

“Only by doing this can we piece together exactly what happened, establish who knew what and understand the role suspects may have played,” he said.

“And as we have always said, the threshold to bring criminal charges is high, so we must be confident that the evidence we present to the Crown Prosecution Service has the best possible chance of meeting this bar.”

Police have interviewed seven more suspects under caution this year, meaning 13 of 53 people under investigation have been questioned. Officers have submitted several files for “early investigative advice”, meaning prosecutors are already helping to build cases.

“However, we cannot underestimate the task in hand,” Clayman said. “Through the many conversations we’ve had with sub-postmasters over the course of our investigation so far, we have been honest about these challenges and the scale of what lies ahead.

“This includes overcoming funding challenges at a time when police forces are already severely stretched. To meet our proposed timeline of submitting files for charging decisions in late 2027/early 2028, we need to double the size of the investigation team from 111 to 210.

“Without this, we risk our timelines being pushed back by as much as five years, which we know is unacceptable for those who have already been living with this for decades.”

The team of police officers and staff across the country was bolstered to more than 100 in 2024, up from 80 initially.

Earlier this year ministers said family members of post office operators affected by the scandal will be allowed to claim compensation under a new government scheme. Close relatives had previously not been eligible under the redress schemes being run by the Post Office and the government.

Williamshad recommended that a scheme for family members be put in place when he published the first volume of his report.

About 3,500 branch owner-operators were wrongly accused of fraud. Across all the redress schemes, more than 11,500 claimants have been paid back a total of £1.48bn so far

Clayman said the police criminal inquiry’s priority was to “deliver justice” for victims and families, and that he had met victims on Tuesday to give an update on the work. Part of that was to “explain some of the challenges we are facing”.

“Many of these victims have been living with the impact of this for 24 years, some have already died and many more are reaching older age,” Clayman said. “Put simply, we do not have the luxury of time and must provide answers as soon as possible to those who so desperately deserve them.”