An investigation is underway after a passenger plane landed at an international airport without clearance.

A controller at Gatwick Airport instructed the pilot of a Boeing 737 to abort a landing at 6.50pm on Monday because the runway was occupied by an aircraft that had already landed, according to the National Air Traffic Services.

The crew of the plane acknowledged instructions to "go-around" but the aircraft continued to land just moments after the other plane left the runway.

Despite the close shave, a NATS spokesman said as the unauthorised plane landed, the runway was no longer occupied and safety was not jeopardised at any time.