Ofgem is to investigate how UK Power Networks and Scottish and Southern Energy (SSE) responded to customers during the Christmas storms, it has announced.
The energy regulator said initial findings showed the two electricity distribution companies had been the slowest to respond to customers and to reconnect their supply after the 2013 storms.
Its review of the electricity Distribution Network Operators' (DNOs) handling of the storms found their speed of response for restoring supply varied significantly.
Around one million customers were without power for at least three minutes, with 95% of them reconnected within 24 hours.
But approximately 16,000 households suffered lengthy cuts of more than 48 hours, and of these more than 500 premises in the UKPN and SSE Southern regions were without supply for more than five days.
Ofgem said: "One in five callers in UKPN and SSE Southern regions decided to hang up before speaking to an adviser whilst Western Power Distribution (WPD) answered calls almost immediately."
Ofgem said it was requiring all companies to review their processes and report back within two months.
The investigation would be concluded in July 2014 and "further action has not been ruled out".
The regulator said it would also consult on increasing payments for customers during severe weather, especially for those disconnected for an extended period of time.
The distribution companies have estimated that 26,000 customers could be entitled to claim £2 million in guaranteed standards payments following the Christmas storms, with UKPN and SSE expected to make 26% and 67% of these payments respectively.
Ofgem's senior partner for distribution, Hannah Nixon, said: "Being without electricity is difficult for customers at the best of times, let alone during a holiday period.
"Whilst we recognise the hard work of the DNOs' operational staff during the severe weather over Christmas 2013, we are very concerned with the variations in the effect of the storms on customers in different areas.
"Our principal duty is to protect the interest of consumers. We will be looking at increasing guaranteed standards payments during severe weather.
"If our further investigation into the performance of UKPN and SSE during the 2013 Christmas storms suggests that these companies have breached their licence conditions, we will undertake enforcement action that could result in a financial penalty."