National Grid operations director Duncan Burt said Friday's blackout was caused when two power stations failed nearly simultaneously, leading the system to cut off power to some parts of the country in order to preserve the rest.
A Government spokesman said the issue is "now resolved".
The Energy Emergencies Executive Committee, carrying out the inquiry, will also explore whether there were any technical problems, plus how well National Grid communicated about the incident and restored the problem.
London's Euston and Kings Cross stations, which are two of the busiest stations in the city and serve the north of England as well as some suburban lines, were closed because of overcrowding caused by the cancellations and delays.
A power cut that affected a million people and caused travel chaos was not the result of a cyberattack, operators of Britain's electricity network said on Saturday.
Blackouts were reported across the Midlands, the South East, South West and North East of England, and Wales.
"However, this time we were kind of unlucky in that it was a deeper drop in frequency so more demand was tripped, and it happened during rush hour", he said.
The company has tweeted saying it is because of a failure on the National Grid network.
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Chancellor Sajid Javid at the National Grid Training Centre near Newark hours before a power cut.
He said alarms were also set off by the temporary outage and traffic lights stopped working entirely.
"All the traffic lights were down, but there were no police present... it was like witnessing something out of an apocalyptic film", Harriet Jackson told Britain's Press Association in south London.
One passenger, Scott McKenzie, said: "It was a bit worrying to start - more so because various alarms were going off and staff clearly didn't quite know what was going on either".
"We are in the process of restoring customers now and will provide further updates as soon as possible for specific areas".
At the worst point of the power cut, about 500,000 people were affected in Western Power Distribution's area - including 44,500 customers in Wales - while 110,000 Northern Powergrid customers also lost power.
He said that there was "no indication" that the power cut was due to a lack of investment in major infrastructure.
The Department for Transport confirmed they were working with Network Rail to reduce disruption.