A price cap on energy bills proposed by the prime minister last week is unlikely to take effect before winter.
Theresa May had vowed to revive a plan to cap charges for an extra 12 million consumers.
However, Ofgem said it would have to wait for legislation to be in force before it could take action on standard variable tariffs.
Until then a more limited price cap will cover another one million low income households, the regulator said.
That move will, on average, save households £120 a year the regulator said.
Last week Business Secretary Greg Clark said a gas and electricity price cap could be imposed as early as this winter should Ofgem decide to use its powers.
The BBC understands that Ofgem is reluctant to do so because it believes energy companies could challenge its decision.
A Department for Business spokesperson said draft legislation on an energy price cap for default tariffs would be introduced this week.
The legislative process could take as long as a year, and then a further five months for Ofgem to implement a cap.
Dermot Nolan, the regulator’s chief executive, told BBC Radio 4’s Today programme that energy firms should be doing more to move people off standard variable tariffs.
“I would challenge the entire industry to say the standard variable tariffs are not good deals. The ball in some senses is in your court – do more,” he said.
Michael Lewis, UK chief executive of E.on, claimed the retail energy market was working well.
“We’ve got 60 competitors in the market with new entrants approaching 20% market share, we’ve got high levels of switching, with 3.5 million customers switching so far this year, which is a significant increase on 2016,” he told Today.
Mr Lewis reinterated E.on’s plans to move all customers off its standard variable tariff from next year.