According to airport sources, poor visibility and non-compliant CAT III B trained pilots led to around 300 delays, 21 diversions and 40 cancellations.
Airport sources told these flights had to be diverted as pilots were not trained to operate in CAT IIIB conditions.
Heavy fog had started to envelop the airport around 2 a.m.
Earlier in the day, an airport source told : "The low visibility conditions lasted from 2 a.m. to 3 p.m. during which 21 flights were diverted... Landings and take-offs were carried out using CAT IIIB Instrument landing system during the period from 2 a.m. to 11 a.m."
"After 11, the visibility improved to over 600 metres and the operations were shifted to CAT II."
The airport has technologically superior CAT (category) IIIB ILS system, which allows compliant aircraft and trained pilots to land even when the runway visibility is just 50 metres.
This year, the new air traffic control tower, thermal imaging cameras and first-of-its-kind 24X7 social media command centre are being used to manage operations during the reduced visibility conditions.
Consequent to fog, private airlines IndiGo and Vistara issued travel advisory for their passengers saying flights to and from the national capital have been impacted due to bad weather and poor visibility.
Dense fog and eventual poor visibility have caused flight delays and cancellations across north India in the past few days.
In a tweet on Monday, IndiGo said: "#6ETravelAdvisory: Due to poor visibility in #Delhi, our flights are impacted. Do keep a track of your flight status."
Vistara also said that its flights to and from the capital have been delayed affecting its other flights too.
"Due to dense fog and poor visibility in Delhi, arrivals/departures from Delhi are delayed with consequential impact across the network," it said.
Earlier on Monday, budget airline SpiceJet also asked its passengers, scheduled to travel to and from Delhi to keep a track of their flight status.
"Due to bad weather at Delhi (DEL), all departures/arrivals and their consequential flights might get affected. Passengers are requested to keep a check on their flight status," the airline tweeted.
( With inputs from IANS )