Seventy-six Conservative MPs have backed calls for Theresa May to press ahead with a cap on energy prices.
In May, Mrs May told the Sun she would, if re-elected, introduce a cap on "unfair" price rises to help 17 million families on standard variable tariffs.
But plans put forward by regulator Ofgem are thought likely only to benefit 2m more households.
A cross-party letter signed by 192 MPs urges the PM to "do more" to protect 15m more households being "preyed on".
The letter to the prime minister and Business Secretary Greg Clark, organised by the Conservative MP and former minister John Penrose, has the backing of scores of Tories including former Work and Pensions Secretary Stephen Crabb and former Environment Secretary Caroline Spelman.
Ofgem already has a cap reducing bills for about four million households that use pre-payment meters to pay for their gas and electricity, introduced in April this year. It is currently consulting on extending that to an estimated 2m more households on low incomes.
The MPs' letter says a price cap was promised by three leading parties at the general election and more needed to be done to protect 15 million households from the "big six energy firms".
"We hope you will work with us and Ofgem to stop this Big-6 stitch-up, and pledge to help the millions of households who Ofgem seem set to ignore."
Mrs May's article in the Sun went further than the Conservative Party manifesto - which pledged only to introduce a "safeguard tariff cap that will extend the price protection currently in place for some vulnerable customers to more customers on the poorest value tariffs".
By the Queen's Speech in June, the government was saying only that it would bring forward "measures to help tackle unfair practices in the energy market to help reduce energy bills".
'Political arm wrestle'
The business secretary has urged Ofgem to use its existing powers to reduce bills.
Conservative MP Mr Penrose told BBC News: "There's a bit of a political arm wrestling going on between politicians in Parliament and Ofgem to work out who gets this done - but somebody's got to."
He explained that the standard variable tariff that he wants to see capped was what most people ended up on - particularly if they did not switch suppliers - and it offered the worst deal.
He said Ofgem's "tiny cap" would only cover a small part of the market and as the Conservatives, Labour and the SNP had all backed a cap "we ought to be able to get on and do it".
Responding to the letter, a spokesman for the business department said: "The government is determined to see the huge detriment suffered by loyal energy consumers addressed.
"The business secretary asked Ofgem to advise on what measures it will take to safeguard consumers and, while we await the regulator's proposals, we remain prepared to legislate if necessary."
Rebecca Long-Bailey, Labour's shadow business secretary said: "Even the prime minister's own party is disappointed by her lack of action on energy prices. They've resorted to publicly urging her to honour the pre-election commitment of a price cap which she has so far failed to deliver.
"Theresa May needs to make good on her words and provide the 17 million customers the cap and £100 saving that she promised."
In August, British Gas said it would increase electricity prices by 12.5% from 15 September - affecting 3.1 million customers. Its owner Centrica said the rise was a result of transmission and distribution costs and the costs of government policy.
Lawrence Slade, chief executive of Energy UK, which represents the major energy suppliers said: "The energy retail market is changing at a rapid pace - as demonstrated by the growth in suppliers to choose from, the ever-increasing switching numbers and the number of people moving off standard tariffs - almost a million in the last six months.
"Energy companies are committed to ensuring a retail market that produces fair outcomes for consumers, including improving support for the most vulnerable. We will continue to work with the regulator to ensure competition can continue to flourish and deliver benefits for all consumers."