Last week, Ofgem said the cap on duel-fuel bills - both gas and electricity - would rise by £117 a year, or 10.3 percent, to £1,254 a year from April 1 for average energy use.

"These changes for standard variable tariffs will not come into effect until 1 April and we will write to customers before then, highlighting the other tariffs available," a spokeswoman for EDF Energy, the British arm of France's EDF, said in an email.

Ofgem was told by parliament last year to set a limit after lawmakers said customers were being overcharged for electricity and gas. Prime Minister Theresa May had called the tariffs a "rip-off".

The regulator last week said the cap would rise from April to reflect higher costs for energy suppliers such as wholesale prices, which it said were 17 percent higher than during the last cap period.

"As a responsible and long-term business, it is important that we reflect the costs we’re facing," the EDF spokeswoman said.

German E.ON's UK subsidiary, another of Britain's large energy suppliers known as the 'big six', said on Monday its prices would rise from April 1 and most of the suppliers are expected to up their prices to match the cap level.

"There is little doubt that this is just the beginning of a crushing wave of price hikes we are expecting across the Big Six," said Victoria Arrington, a spokeswoman for energy switching site energy helpline.

Britain's other big six energy suppliers are Centrica's British Gas, SSE, Iberdrola's Scottish Power and Innogy's npower.

(Reporting by Susanna Twidale; editing by Jason Neely and Emelia Sithole-Matarise)

By Susanna Twidale