Italian firm Eni has agreed to sell a 30% stake in the Shorouk concession offshore Egypt to Russian oil company Rosneft for $1.13billion.
Rosneft also has an option to buy a further 5% stake.
The concession contains the Zohr field, the largest natural gas deposit ever found in the Mediterranean, with an estimated 850billion cubic meters of gas in place.
It is expected to produce gas by the end of 2017.
Eni currently holds 90% of the block, having recently agreed to sell 10% to BP.