Most parts of North India are reeling under severe power outage since early morning today, due to collapse of Northern electricity transmission grid.

The collapse of the Northern power grid impacted public transport systems including, Railways and Delhi Metro, while normalcy is likely to return by afternoon.

"The fault is not known as yet... somewhere near Agra, a failure has happened. We will enquire that," Power Minister Sushilkumar Shinde said.

Even though the exact reason for the grid failure is yet to be ascertained, official sources that many lines tripped and there could be even be the possibility of over withdrawal of power by some states.

"A lot of load has been restored, especially for the essential services such as Railways. The connectivity of thermal plants (supplying power to Northern region) to the grid is expected to be fully restored in the next four to five hours," POSOCO Chief Executive Officer S K Soonee told PTI.

Power System Operation Corporation Ltd (POSOCO), part of state-run Power Grid, manages the transmission grids in the country.

"Emergency transmission services are being restored. The work is on and in a few hours, the National Grid would be completely restored," Power Secretary P Uma Shankar told PTI.

According to officials, presently, electricity from Eastern and Western grids are being diverted to Northern grid.

The Northern Grid, which is estimated to cater to around 28 per cent of country's total population, had failed due to dense fog back in January, 2010.

The Northern Grid, one of the largest in the country, covers nine places -- Punjab, Haryana, Rajasthan, Delhi, Uttar Pradesh, Uttarakhand, Himachal Pradesh, J&K and Chandigarh.

As per the National Load Despatch Centre (NLDC), Northern Grid was affected at around 2.35 am today morning.

NLDC is the apex body to ensure smooth functioning of the national power system.