Political pressure surrounding CNOOC’s $18.5bn unsolicited bid for Unocal, the California oil company, was stepped up on Thursday as Washington lawmakers overwhelmingly supported a measure aimed at blocking the bid by cutting off funds for the Bush administration to investigate the deal.

The 328-91 vote in the House of Representatives, on an amendment by Carolyn Cheeks Kilpatrick, a Democrat from Michigan, would stop the Treasury Department, which chairs the committee that reviews foreign acquisitions of US assets on national security grounds, from using federal funds to recommend an approval of CNOOC’s bid.