This marks the end of a protracted legal case, which is a victory for more than 80,000 members of section C of the BTPS that would have stood to lose an estimated £24,000 each on average from the switch, according to Prospect's analysis.
The Retail Price Index will continue to be used by the scheme to index member’s pensions.
The legal case was based on the wording of the rules of Section C of the BTPS – particularly the requirement for RPI to have “become inappropriate” for the purpose of increasing pensions before it can be changed for another index.
Prospect national secretary Noel McClean said: “We welcome this decision by the Supreme Court. Had BT succeeded it would have had the potential to cut the value of members' pensions in retirement by many thousands of pounds. This is a victory for members of Section C of the BT Pension Scheme that finally brings this matter to a close after a long and protracted legal battle”.