A bag which caught on fire as it was loaded onto an aeroplane was set alight by a vape pen battery, it has emerged.

Baggage handlers noticed smoke wafting from the bag while stowing hold luggage from Seattle-Tacoma International Airport onto a United aircraft.

Firefighters arrived on the scene to tackle the flames and the owner of the luggage was swiftly escorted away to identify the bag.

Baggage handlers noticed smoke wafting from the bag while transporting hold luggage from Seattle-Tacoma International Airport onto a United Flight (stock image)

It was determined that the fire stemmed from a battery connected to a charger for a vape pen, Komo News reported.

The flight to Houston from Seattle was delayed slightly as a result of the incident.

A similar episode occurred earlier this year when a Delta flight was delayed after an electronic cigarette in a passenger's carry-on bag ignited while the aircraft was still on the tarmac.

Fortunately the flames were 'safely extinguished with no damage to the aircraft', a spokesperson said, and no passengers needed to be evacuated.

The U.S. Department of Transportation banned the use of electronic cigarettes on commercial flights in April.

The U.S. Department of Transportation banned the use of electronic cigarettes on commercial flights in April (stock image)

According to United's vape policy: 'Electronic cigarettes and personal vaporizers are not accepted in checked or gate-checked baggage.

'These items may be stowed in carry-on baggage or on your person during travel, however, the use and charging of e-cigarettes and personal vaporizers is prohibited onboard all United flights.'

The FAA is now investigating the incident.