Under the terms of the contract BP Gas Marketing will take IOG's 50% share of the gas produced from the forthcoming Blythe field in the southern North Sea.
The gas is expected to be sold at the Bacton terminal on the Norfolk coast.
Shares in IOG surged more than 8% and ended the day 2p ahead at 26.5p.
The Blythe field is operated by ATP.
Yesterday IOG said it expects a development plan for the field to be submitted by the third quarter of this year.
Mark Routh, chief executive of IOG, said: "This agreement is another important step on the road to first gas from Blythe and also builds upon our excellent relationship with BP."
Mr Routh, who sold CH4 Energy to Aberdeen based Venture production for more than £154 million in 2006, highlighted a crude oil agreement IOG has already signed with BP Oil International for the Skipper field.
Chris Schemers, head of origination at BP Gas Marketing, said: "As a North Sea producer BP understands the challenges faced by explorers and producers in getting their gas to market. We're pleased that many independent producers choose us to flow their gas into Europe and the UK."