A US passenger plane was forced to turn back in mid-air because a human heart had been left on board.

The Southwest airlines flight from Seattle to Dallas was in the air for three hours before the organ was discovered.

It had been due for collection in Seattle, but was never removed from the cargo-hold.  

The pilot informed shocked passengers that the flight had to return urgently to preserve the heart for a valve-transplant.

The plane spent three hours in the air – landing 12 hours before the time-limit on its viability for use would have expired, according to the Seattle Times.  

The heart was not intended for any particular patient, and upon landing was delivered by a contracted courier to LifeNet Health, a tissue processor in the city of Renton. 

A doctor among the passengers told the paper the incident was a “horrific story of gross negligence”.

Southwest, which uses the slogan "We run on Southwest Heart", said in a statement: "We made the decision to return to Seattle as it was absolutely necessary to deliver the shipment to its destination in the Seattle area as quickly as possible."