BT Group, which reports third-quarter results on Thursday, could see its pension deficit balloon to £4.6bn from £400m if it were required to report its retirement liabilities and assets under new rules proposed last week by the Accounting Standards Board.

In a report for RBC Capital Markets, John Ralfe, an independent pensions consultant, notes that the proposed rules do not alter the underlying economics of BT’s pensions obligations; they simply make them more transparent.