Judges today told BT it cannot change the way it calculates pensions increases for 80,000 current and past employees.
The telecoms provider, which links increases to the retail prices index (RPI), was hoping for permission to start using a different measure.
BT, which has one of the biggest pension schemes in the UK, argued that the consumer prices index is a better indicated of price changes.
But the Court of Appeal found the company is unable to change the index, a move which union Prospect estimates would cost the average pensioner £24,000.
BT said it was considering what to do next.
A spokesperson said: “The High Court decided in January that it was not possible to change from RPI to another index. We appealed that decision.
“The Court of Appeal has now handed down its judgment which upholds the High Court’s decision. We are disappointed with the outcome and will now consider the judgment in detail in order to decide next steps.”
The case concerns around 83,000 past and current employees on the company’s section C pension, who joined the company between 1986 and 2001. It does not affect increases to its section A and section B employees, which are already calculated using CPI.
Prospect national secretary Noel McClean said: “We welcome the Appeal Court’s decision to uphold the original High Court ruling.
“BT was seeking to cut the future incomes of BT pensioners and current employees by tens of thousands of pounds in order to transfer an estimated £2bn to shareholders, and Prospect does not believe this was acceptable.”