The woman who is in her 40s and presumed to be from an Eastern European country, managed to sneak past security and board the flight bound for London, Gatwick.
According to Norwegian, the crew alerted Spanish police when they found an extra person on-board during a routine head count. However, Britain's The Sun newspaper reports that a passenger alerted the crew after seeing the woman flaunt several passports.
“We will not speculate on what happened before our crew discovered the woman inside the plane, but our routine procedure exposed that there was a person with no documentation on board,” Tonje Naess of Norwegian told Norway's Nettavisen news site.
The plane was evacuated immediately as armed police entered the plane to take the woman off.
“It was terrifying. We didn’t know if she was a terrorist. Police with guns arrived and there was a rush to get off. A mum and baby got squashed,” pregnant passenger Julia Hilton told the Sun.
The plane's passengers re-boarded after 90 minutes, causing a two-hour delay.
“We take such incidents seriously and although it happens very rarely our crew is well trained to deal with situations like this. They acted professionally, acted in accordance with procedures and police were called,” Naess told Nettavisen.