Customs officers at Heathrow Airport were told not to stop suspected drug smugglers in the run-up to Christmas because of a lack of resources.
Staff were asked to help in "managing this situation", and instructed not to "actively seek to identify any passenger with internal concealments".
But UK Border Agency (UKBA) chief Jonathan Sedgwick has admitted the e-mail should not have been sent.
He added a review would be held so the matter "does not arise in the future".
The instruction covered the period from 23 December to Christmas Day.
But Mr Sedgwick, UKBA deputy chief executive, said the message did not represent the organisation's policy.
He said: "Our aim is to prevent and detect the smuggling of drugs however people try to bring it into the country.
"Our drug enforcement teams have worked tirelessly over the Christmas period and have successfully intercepted several shipments of class A drugs."
Heathrow, one of the world's busiest airports, carries about 70 million passengers a year.