An Air New Zealand flight from Los Angeles to London had to divert to Iceland because of a medical emergency.

NZ2 was en route to Heathrow yesterday when it had to make the unscheduled stop

The aircraft, a Boeing 777-300, was just off the tip of Greenland when it changed course to Keflavik Airport in Reykjavík, the capital of Iceland.

Air New Zealand said the plane diverted when a passenger became unwell.

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''The customer was disembarked to receive medical care and the flight was refuelled and then continued on to London around 1 hour 30 minutes late,'' a spokeswoman said.

Data on FlightAware shows the plane landed eight hours into a flight which usually takes around 10 hours

Keflavik International Airport was built by the US military during World War II. It has played a big role in the development of commercial flight in Iceland and been an important stop-over between Europe and the US but is increasingly a destination airport for the growing number of airlines serving the country as tourism booms.

Diversions are costly for airlines.

Last year Emirates said a single flight diversion could cost it from US$50,000 ($75,000) to over US$600,000, depending on its nature.

That includes fuel, flight catering, landing and ground handling fees, air navigation cost, passenger rebooking costs and onward connection, as well as other associated costs to care for crew and passengers.

The Washington Post last year reported on the incidence of in-flight medical emergencies following a study in the New England Journal of Medicine.

Researchers estimated that of the 2.75 billion total passengers who flew each year in the two years between 2008 and 2010, there were 44,000 in-flight medical emergencies. That boils down to one in every 604 flights, or nearly 50 every day.

In those cases, the most common symptoms included fainting or lightheadedness (37.4 per cent), respiratory issues (12.1 per cent), and nausea or vomiting (9.5 per cent). Just over 7 per cent of affected flights landed because of the illness.