Two more energy suppliers, E.On and British Gas, are to freeze their standard tariffs for the coming winter.
The move, for both gas and electricity bills, will affect around nine million customers in total.
Both suppliers have promised that standard tariff customers will now see no increase in charges until at least April 2017.
The decision follows similar action from SSE, which said it would cap standard tariffs for the same period.
It comes amid mounting political pressure on the big suppliers to treat loyal customers more fairly.
British Gas's announcement was made in a series of adverts in national newspapers. It said its standard tariff would "remain unchanged through the winter", which is understood to mean at least until the end of March 2017.