The policy will apply to those travelling on Basic Economy tickets upwards but excludes extra legroom Economy Plus seats. It will see passengers travelling with children under 12 offered adjacent seat options following a technology upgrade that dynamically finds available seats at the time of booking. 

The online seat engine first reviews all available free economy seats and then opens complimentary upgrades to available Preferred Seats if needed.

United said: “In instances when adjacent seats are not available prior to travel - due to things like last minute bookings, full flights or unscheduled aircraft changes – United’s new policy also lets customers switch for free to a flight to the same destination with adjacent seat availability in the same cabin.

"Customers also won’t be charged if there is a difference in fare price between the original and new flight.”

United’s Polaris business class and United First Class are excluded from the policy change.