BT had said it was reviewing a possible switch to the consumer price index (CPI) from the retail price index.

The CPI is generally lower, meaning the move could have helped BT tackle a deficit that stood at 9.3 billion pounds on Sept. 30.

"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 in a statement.

BT already uses the CPI index to calculate pension increases for the more than 200,000 members of two other pension schemes.

BT shares fell after it announced the ruling. They stood at 263.6 pence at 1100 GMT, down 1.27 percent.

(Reporting by Paul Sandle; editing by Jason Neely)