"No flights were taking off or landing at the airport as the visibility reduced to below 50 metres due to the dense fog which began to descend at around seven am," airport sources said.

Airport authorities were caught unawares as fog suddenly enveloped the airport after a clear morning. More than 40 flights departing from here were delayed by over two hours while four SpiceJet flights from Mumbai, Pune, Kolkata and Ahmedabad scheduled to arrive here at 7.45 am were diverted to Jaipur. Most flights arriving in New Delhi were delayed by over two hours.

The fog reduced the runway visibility to less than 50 metres.

Early morning flights went as per schedule as the visibility was above 1,500 metres and general visibility was 100 metres at around 6.30 am.

All flights which were scheduled to take off after 7.30 am were badly affected, as passengers had boarded and doors were closed when the fog descended.