Police reportedly believe that Mustaf Jamma, a prime suspect in the fatal shooting of Pc Sharon Beshenivsky, used his sister's passport and wore a full niqab to evade checks at Heathrow airport.
Shadow Home Secretary David Davis insisted there should be an urgent inquiry.
At the time of the 26-year-old's apparent escape between Christmas Day last year and New Year's Day, he was one of the UK's most wanted men and the airport was on high alert in the wake of the July 7 bombing attacks.
But it is thought that staff rarely ask those departing Britain to remove veils in order to make sure their identification is valid.
A BAA Heathrow spokesman told The Times: "Immigration control staff always do a visual check on people coming into the country but only random checks are made with outgoing passengers."
Via Tim Blair.