Flight operations at the Indira Gandhi International airport here came to a standstill with the general visibility falling below 50 metres and runway visibility range (RVR) dipping to 125 metres, lower than the required minimum of 150 metres for the planes to take off, sources at the airport said.

Around 40 outgoing domestic flights and 15 incoming ones were delayed by up to five hours, while 11 international flights arriving at the IGI airport and 10 departing from there were also delayed, the sources said, adding that 18 flights were cancelled.

Several trains approaching the national capital were running several hours late, with some behind schedule by up to 20 hours, railway sources said.

The trains running late included Goa Express, Mumbai- Firozpur Express and Vaishali Express.

North-east Express and Dibrugarh Rajdhani from Delhi have been rescheduled, they said.

Barring a few, most of the early morning flights were either delayed or cancelled as the fog began enveloping the IGI airport from 1.30 am.

"The visibility dropped to below 50 metres and the RVR also dipped to a low of 125 metres. The visibility at the airport will remain under CAT-III B conditions, that is below 150 metres till noon," R K Jenamani, Director at the airport's Met Department said.

The airport has been experiencing dense fog covering both the runways since this morning, forcing the airport authorities to implement low visibility procedures (LVP).

The LVP is generally implemented when the RVR dips below 800 metres. The passengers were worst hit due to the cancellation of flights. Most of them complained that they were not informed by their airlines regarding delays or cancellations.

However, airline officials refuted these claims, saying they had sent SMSes but due to network congestion on New Year's day, the messages probably took time to reach the passengers.

The airport has been facing thick fog for the third day on Thursday in this week.