The criminal investigation into the Post Office Horizon IT scandal could be delayed by five years unless it receives millions of pounds in extra funding, police chiefs have warned.
The commander leading the national police inquiry, Stephen Clayman, said the size of the investigation team would need to double to meet its current timeline of submitting files for potential prosecutions by late next year or early 2028.
He said 111 detectives were already working on a "hugely complex" investigation but another 99 were needed.
A government spokesperson said the scandal was "an appalling injustice" and that it was "considering requests for further funding".
Clayman said a delay would be "unacceptable for those who have already been living with this for decades".
The Horizon IT system, which began operating in 1999, falsely created accounting shortfalls in Post Office branches for which sub-postmasters were held liable.
The scandal has been called the UK's most widespread miscarriage of justice.
More than 900 people were prosecuted and some went to prison. Some died while waiting for justice.
The criminal investigation into the scandal, Operation Olympos, began in 2020.
It's now a joint national police investigation between the National Police Chiefs' Council and the Metropolitan Police Service, with involvement from police forces across the UK.
The bulk of the police investigation is being paid for by individual forces, but it also relies on grants from the Home Office.
Clayman said £2.8m had been received from the Home Office, but this was £16.5m short of what was needed for this financial year to boost the number of detectives.
"It's very worrying," said Seema Misra OBE, a sub-postmaster who was jailed when pregnant in 2010 after wrongly being accused of stealing £74,000 from her branch in Surrey.
"How can the government spend hundreds of millions of pounds on lawyers dragging this out but it's different for the common people to get justice? We need accountability," she told the BBC.
Clayman said seven more suspects had been interviewed under caution this year, taking the number of people who've now been questioned to 13 out of a total of 53 individuals currently under investigation.
He added detectives were dealing with some eight million documents and growing, with many of them needing to be forensically reviewed.
"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.
"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.
"We cannot underestimate the task in hand. Through the many conversations we've had with sub-postmasters over the course of our investigation so far, we have been honest about those challenges and the scale of what lies ahead."
He said overcoming funding challenges comes at a time when police forces were already "severely stretched".
A government spokesperson said: "It is important that victims' voices are heard and that the causes identified through the public inquiry, and full and fair redress is paid out quickly to those who suffered."
