UK energy price cap could be in place by Christmas 2018

(Updates throughout with detail)

LONDON, Jan 10 (Reuters) - A government plan to cap the most common form of gas and electricity tariffs for millions of British households could come into effect in time for Christmas 2018, regulator Ofgem said on Wednesday.

Prime Minister Theresa May said in October she would impose controls to tackle what she called "rip-off energy prices" - home power bills have doubled in Britain over the past decade to an average of about 1,150 pounds ($1,500) a year.

Dermot Nolan, Ofgem chief executive, said for the measure to take effect by Christmas it would have to receive royal assent - meaning it officially becomes law - before Britain's lawmakers break for summer recess on July 20.

He was speaking at a cross-party parliamentary committee.

Once the law is passed, Ofgem would need to launch a statutory consultation process of around 50-60 days and then allow energy suppliers further time to implement the measure.

Ofgem is charged with setting the cap on so-called standard variable tariffs (SVTs), the most commonly used deals, and the basic rate that energy suppliers charge if a customer does not opt for a specific fixed-term deal.

Data published by Ofgem in December showed among Britain's "Big Six" energy suppliers, SSE (LSE: SSE.L - news) had the largest percentage of customers on SVTs at 71 percent.

British Gas, owned by Centrica (Frankfurt: A0DK6K - news) , had 67 percent of its customers on SVTs and E.ON had 61 percent.

Britain's other three main suppliers are EDF Energy , Innogy's Npower and Iberdrola (Amsterdam: ID6.AS - news) 's Scottish Power.

The Big Six are poised to become the Big Five, with SSE and Innogy planning to merge their retail power and gas operations.

($1 = 0.7383 pounds) (Reporting by Susanna Twidale; Editing by Mark Potter and Dale Hudson)