Lufthansa, which is offloading the loss-making carrier to British Airways, on Wednesday told existing bmi pensioners and employees that changes were being made to the final salary scheme ahead of the takeover, which closes on Thursday.
The final salary pension scheme is being entered into the Pension Protection Fund (PPF) - an option normally used for companies that go bust.
The trustee of the scheme, Clive Grimley, admitted in a letter the move would "result in members receiving compensation that is less than their full expected benefits".
Bmi employees who are yet to retire will now only receive 90pc of their final salary pension. Payments will also be capped depending on their age at retirement.
Lufthansa has agreed to pay £84m to make up for some of the lost benefit, which will be distributed according to "individual members' characteristics".
Balpa, the pilots' union, said: "This will have a direct and negative effect on pilots' pensions."
A Lufthansa spokeswoman said the PPF was a last resort after the Pensions Regulator rejected a previous proposal.
Bmi pensions to be cut after sale to BA - Telegraph
BMI pensioners face cuts to benefits - FT.com