India's capital New Delhi was enveloped in heavy, toxic smog on Sunday - the worst levels in recent years - with flights diverted or delayed as politicians blamed each other for failing to tackle the crisis.

Every winter, the megacity of 20 million people is blanketed by a poisonous smog of car fumes, industrial emissions and smoke from stubble burning at farms in neighbouring states.

Concentrations of particles measuring less than 2.5 microns hit the highest level of this season, exacerbated by light rains late Saturday, India's state-run System of Air Quality Weather Forecasting and Research (SAFAR) said.

The reading for pollutants in the atmosphere hit 810 micrograms per cubic metre Sunday morning, beyond the “hazardous” zone according to the US embassy in Delhi, which independently monitors pollution levels.

The recommended World Health Organisation safe daily maximum is 25.