Post Office campaigner Sir Alan Bates is yet to receive a reply from Sir Keir Starmer, despite writing to him over a month ago.
Sir Alan said he had written to the prime minister to remind him the "clock is still ticking" on a financial redress deadline for victims.
In his letter, he demanded a March 2025 deadline for compensation for sub-postmaster victims of the Horizon scandal.
Sir Alan confirmed to Sky News he was yet to hear back from the prime minister.
"It was over a month ago," he said.
"I sent him a reminder yesterday. I told him the clock is still ticking and it's now five months from the March deadline, which I'm told is still achievable by other professionals.
"So let's get on with it, that's all we want. Get on with it."
Sir Alan appeared before the Business and Trade Select Committee to answer questions on apparent delays experienced by victims using various compensation schemes.
He described issues including "shifting goalposts" within the "guidance and principles" underpinning the schemes, saying they "have changed on a number of occasions".
He also said the schemes were often "bogged down" in bureaucracy.
Sir Alan emphasised the issue of "fairness" in the schemes - questioning whether it is the government or victims who decide.
When asked if his offer was "fair", he replied: "In whose eyes?"
He also said potential legal action against the government "is a consideration" if they do not deliver on redress by March next year.
He raised concerns, however, that it might "halt" the current Group Litigation Order (GLO) scheme but said it "might be a choice that people are prepared to take".
"It's the deadlines that have to be set," he added. "They [the Department for Business and Trade] are terrified of having deadlines."
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(c) Sky News 2024: Post Office campaigner Sir Alan Bates says he is yet to receive reply to letter to PM