Sir Alan Bates has accused the government of putting forward a "take it or leave it" offer of compensation amounting to less than half of his claim.
The former subpostmaster, who has campaigned for redress for victims of the Post Office scandal, said ministers were presiding over "quasi-kangaroo courts" and altering the "goal posts" on their judgements.
Writing in the Sunday Times, external, Sir Alan said: "Claims are, and have been, knocked back on the basis that legally you would not be able to make them, or that the parameters of the scheme do not extend to certain items."
The Department for Business and Trade said: "Anyone unhappy with their offer can have their case reviewed by a panel of experts."
Between 1999 and 2015, more than 900 subpostmasters were wrongly prosecuted after the faulty Horizon IT system indicated shortfalls in Post Office branch accounts.
Sir Alan led a group of 555 subpostmasters who took part in the landmark group legal action against the Post Office.
Their fight for justice was launched into the national consciousness last year by an ITV drama about the scandal, Mr Bates vs The Post Office.
The government went on to set up a specific compensation fund to ensure these subpostmasters received extra money to reflect the gravity of their situations, but progress has been described as slow and many are still waiting for a payout.
Under the Group Litigation Order (GLO) scheme, claimants can either receive £75,000 or seek their own settlement.
How do the Post Office scandal compensation schemes work?
Sir Alan said promises that compensation schemes would be "non-legalistic" had turned out to be "worthless".
He has also called for an independent body to be created to deliver compensation schemes for this and similar public sector scandals.
As of January, £128m had been paid under the GLO scheme, including interim payments, according to government data, external.
A Department for Business and Trade spokesman said: "We pay tribute to all the postmasters who've suffered from this scandal, including Sir Alan for his tireless campaign for justice, and we have quadrupled the total amount paid to postmasters since entering government.
"We recognise there will be an absence of evidence given the length of time which has passed, and we therefore aim to give the benefit of the doubt to postmasters as far as possible. Anyone unhappy with their offer can have their case reviewed by a panel of experts, which is independent of the government."