United Airlines has issued an apology after “inadvertently” reintroducing flights numbered 93 and 175 – the same as those involved in the 9/11 terror attacks.

Airline unions described the move as “a terrible misstep” by airline unions.

Greg Davidowitch, president of AFA at United, said: “We have advised United Airlines that reinstatement of flights 93 and 175 is a terrible misstep. Out of respect for those who lost their lives and those who will always remember our heroes, United Airlines must do what should have already been done – immediately and permanently retire these flight numbers.”

A spokesman for United said: “We regret that these flight numbers were inadvertently reinstated in our system. We are taking immediate steps to remove them and apologize for the error.”

A recent reshuffle as part of the ongoing merger of United Airlines with Continental Airlines saw the numbers reintroduced as codeshares. Although the flights were operated by Continental they were sold under the United brand.

United 175, Boston-Los Angeles, was flown into the south tower of the World Trade Center. United 93, Newark-San Francisco, crashed into a Pennsylvania field, killing 33 passengers, seven crew and four hijackers

The airline subsequently withdrew UA93 and UA175 from use.

Neither of the flight numbers would have been used on the same route but the airline responded quickly to complaints.

Air France changed the number of its Rio-Paris flight from A447 to AF445 following the crashed over the Atlantic in 2009.