Origin Energy has formally rejected a hostile A$13.8bn (US$13.3bn) takeover bid from BG Group amid widespread expectations that the UK gas producer will be forced to raise its offer if it wants to secure the Australian group’s huge coal-bed methane reserves.

All of Australia’s major broking houses are valuing Origin at more than the bid price, with Credit Suisse and Merrill Lynch valuing the energy retailer at A$17.8bn and A$15.5bn respectively.