China National Offshore Oil Corporation has launched a bid worth almost $20bn for Unocal of the US, setting the stage for the largest contested takeover battle involving a Chinese company.

CNOOC's offer, decided at a six-hour board meeting in Beijing on Wednesday, aims to trump an agreed takeover by Chevron. The move by the state-controlled group underlines the global ambitions of big Chinese companies. It will also be a test of how open US regulators and politicians are to attempts by Chinese companies to make US acquisitions.