Mr Rogerson, 53, departs with a pounds 200,000 pay-off and a pounds 149,000 company pension. He was on a fixed-term contract with BG until the end of 1999 with a basic salary of pounds 320,000.
He is to become non-executive chairman of a BG subsidiary, Pipeline Integrity International, which the group yesterday said it had sold to Mercury Asset Management's Private Equity arm for pounds 90m. Pipeline Integrity, based in Cramlington, Northumberland, is responsible for high tech inspections of the gas pipeline network using equipment nicknamed the "intelligent pig".
Mercury has raised pounds 116m from private investors and in debt to fund the purchase, which includes a deferred payment of pounds 6m to BG dependent on the company's performance over five years. Tony Powell, from Schlumberger, has been drafted in as chief executive.
Mr Rogerson was responsible for piloting BG's complaint against pipeline cuts through the Monopolies and Mergers Commission (MMC), which last year rejected much of the the company's case. He had previously said he would leave BG after the MMC probe concluded and has already become non-executive director of the Halifax and deputy chairman of Aggreko, the power hire specialists.