Centrica, which has former Enterprise Oil boss Sam Laidlaw as its chief executive, said its 845 pence per share offer is final unless a rival bidder emerges.
The utility controls 29.9% of Venture's shares and this position will not change until a formal offer document is filed in the coming weeks and Venture's board has formally responded.
Four months ago, Centrica acquired a 22% stake in Venture at 725 pence per share.
