British Gas and Scottish Power have followed EDF Energy, Eon and Npower to increase prices to the maximum permitted under the new price cap.
Customers face a 10% increase in bills from 1 April. British Gas customers will see on average £119 a year extra added to their bills, while the increase will add £117 a year to Scottish Power customers' bills.
The rise affects the four million customers on the standard British Gas tariff and 900,000 customers on Scottish Power's standard tariff. If you're one of them, you could save up to £348 a year compared with its new prices by switching to the cheapest deal on the market.
The price increase will affect your bills if you're on one of the companies' standard or default tariffs. The standard tariff is a variable deal, with no end date.
It will also mean that British Gas can charge more when moving customers on to its future default tariffs, called 'temporary' tariffs.
You're likely to be on the standard tariff if you haven't switched energy company or tariff. You'll be put on to British Gas' temporary tariff if you take no action when your fixed deal ends.
Customers already on a British Gas temporary tariff won't be affected; this is because they are one-year fixed deals. But customers moved automatically on to a temporary tariff from 1 April could face higher prices.
Neither type of tariff charges exit fees if you want to leave, so you can switch to a cheaper deal, penalty-free, before the price rise takes effect. You have until 1 April to switch.
The cheapest deal on the market today costs £906 a year for a household that uses a medium amount of gas and electricity. Switch to it from British Gas' standard tariff (or a deal from another company set to charge the level of the price cap) and you could save £348 in a year.
Currently there are 18 energy tariffs available across England, Scotland and Wales that cost less than £1,000 a year for a medium user.
To keep your options open, look for a tariff with no exit fees. Then if cheaper deals appear in a few months' time, you can switch again without any penalties.
Customers on standard and default tariffs are currently protected by a price cap on energy bills, meaning the amount the company can charge you on these tariffs is limited. Currently, the cap for a medium energy user is £1,137 a year. This is an illustration; your actual bill depends on how much you use.
In response to the price cap increase, four of the biggest energy firms have raised their prices.
Alex Neill, Which? managing director of home services, said: 'Energy customers will be feeling the heat as these latest increases mean price hike Monday is set to see more than one billion pounds added to household bills across the country.
'If you are one of millions of customers stuck on a company's bad value standard variable or default tariff, our advice is simple, switch.'
Prices are also increasing for customers who top up prepayment meters for gas and electricity.
British Gas customers will pay, on average, £107 extra (or 9%) a year from 1 April.Scottish Power customers will pay an extra £106 a year.
Prices are based on a dual-fuel tariff available in all regions in England, Scotland and Wales paying by monthly direct debit, with paperless bills.
Energy use is based on Ofgem's annual average figures for a medium user (12,000kWh gas and 3,100kWh electricity). Data is from Energylinx. Prices given are averages across regions, are rounded to the nearest whole pound and correct on 19 February 2019.