Telecom giant BT Group said the UK's High Court had rejected its request to change the index it uses to calculate certain pension increases.
The company wanted to switch from using the retail prices index, or RPI, to another index.
"We are disappointed with the decision and will now consider the judgment in detail in order to decide next steps, including the possibility of an appeal," BT said.
Separately, BT said a triennial pension valuation was ongoing and that it still expected to complete the valuation in the first half of the 2018 calendar year.
"We continue to review the future pension benefits under our main defined benefit and defined contribution schemes in the UK, with the objective of providing fair, flexible and affordable pensions," BT said.
"We have just completed a consultation with our affected employees and are now considering their feedback before concluding the review."
At 1:11pm: (LON:BT.A) BT Group PLC share price was -4.97p at 262.03p