Energy firm EDF is raising the price of electricity for the second time this year for customers on standard tariffs.
From 21 June, EDF customers will see electricity prices increase by 9% and gas prices go up by 5.5%.
The supplier previously increased electricity prices by 8.4% on 1 March, although it cut gas prices in January.
The rise was immediately criticised by the energy regulator, Ofgem, which said it was hard to justify.
The average dual-fuel customer will pay an extra £78 a year, bringing the annual bill to £1,160, EDF said.
Combining both increases will mean that 1.5 million EDF customers will pay 18.1% more for their electricity this year.
However, taking into account the previous cut, gas prices will remain flat, the company said.
Most of its customers - 1.8 million - are now on fixed price deals, so will not be affected.
Ofgem was highly critical of the increases.
Dermot Nolan, its chief executive, said: "EDF's second price rise in four months, when there has not been a dramatic rise in wholesale energy prices since it last put up prices, is difficult to justify and is further evidence that the energy market is not working in all consumers' interests."
Commentators said EDF customers on standard tariffs could save more than £300 a year by switching to the best fixed-price deal.
"EDF has hit millions with a second shock price hike," said Archna Luthra, head of energy at MoneySavingExpert.
"If you haven't compared and switched already, see this as a final red alarm bell to take action - what are you waiting for?"
EDF said it would be contacting all customers on standard variable tariffs to review their situation.
The company is also introducing a new three-year fixed deal.
"I know that price rises are never welcome, but the industry is facing significant cost increases," said EDF's chief executive, Vincent de Rivaz.
"To be a sustainable and responsible business, we aim to make a fair margin in supplying customers. This fair margin allows us to invest for the long term, in particular in good service, innovation and smart metering."
Five of the big six suppliers have announced price rises this year:
- Npower increased its electricity prices by 15% and gas prices by 4.8% on 16 March
- E.On is due to put up electricity prices by 13.8% and gas prices by 3.8% on 26 April
- Scottish Power raised its electricity prices by 10.8% and gas prices by 4.7% on 31 March
- EDF, combining previous prices changes with those in June, will raise electricity prices by 18.1%, but gas prices will stay the same
- SSE will raise electricity prices by 14.9% on 28 April, although gas prices will be unchanged
- British Gas has announced a price freeze until August.