The Bradford & Bingley Action Group is threatening legal action against the Treasury after the Government's independent valuer told 2,000 investors they will not get any compensation.
Many investors are Yorkshire pensioners who bought permanent interest-bearing shares when B&B was still a building society.
In a letter to Gordon Brown, the BBAG said: "The failure of your Government and its various agencies to provide full details of the sequence of events in the period immediately prior to and post nationalisation places BBAG at a considerable disadvantage in relation to the compensation process generally.
"The valuer of B&B will have access to relevant information which will not be made available to the former shareholders and bondholders of the bank.
"This will make it impossible for those stakeholders to determine whether or not (the] valuation is fair and proper. In our view that is a significant defect in the compensation process."