A price cap on energy bills proposed by the prime minister last week is unlikely to take effect before winter.
Theresa May had promised to revive a plan to cap charges for an extra 12 million consumers.
Howewer, Ofgem said it would ensure that all consumers on standard variable tariffs would be protected as soon as possible "if legislation is in place".
Until then a more limited price cap will be extended to another one million low income households, Ofgem said.
Restrictions on the cost of gas and electricity for those with pre-payment meters already saves some four million households about £80 a year.
The regulator wants to extend the scheme to about two million other people who get certain benefits.