Anti-narcotics co-operation between the US and Mexico could be set back three decades if Mexico’s lower house of Congress approves a bill limiting the activities of “foreign agents”, including the US Drug Enforcement Administration.

William Barr, the US attorney-general, has warned that the bill — which passed the Senate on December 9 and was expected to be approved by the lower house before it breaks for Christmas on Tuesday — “can only benefit the violent transnational criminal organisations and other criminals that we are jointly fighting”.