Theresa May is facing an outcry from 53 Tory MPs after dropping her pledge to introduce an energy price cap for 17 million families "preyed on" by the Big Six suppliers.
The MPs, led by former minister John Penrose and including former cabinet members and ex-leader Iain Duncan Smith, called on the Prime Minister to fulfil her promise to introduce a cap for all families on standard variable tariffs.
The scale of the protest, which has the backing of 38 Labour and SNP MPs, threatens to cause trouble for Mrs May, who is now leading a minority government following a disastrous general election.
The Conservatives promised before the vote to introduce a cap which aimed to save the families up to £100 a year on their bills but the plan did not appear in the subsequent Queen's Speech, prompting accusations of a U-turn.
Energy regulator Ofgem is now considering introducing a price cap on bills aimed at protecting around 2.5 million vulnerable consumers.
The MPs' challenge comes weeks after British Gas hiked electricity prices by 12.5% for 3.1 million customers.
In a letter to the PM, they said: "We are writing to urge you to extend Ofgem's proposals of 3 July 2017 by introducing an energy price cap that protects all of the 17 million families currently on expensive standard variable tariff deals, not just the two million vulnerable ones.
"While these proposals are a step in the right direction, it is clear we must do more to protect the further 15 million households who continue to be preyed on by the Big Six energy firms.
"As you can see from our signatures below, the idea has substantial cross-party support.
"It was promised in the three leading party manifestos and a temporary, relative price cap has support from most of the 'challenger' energy firms - the insurgents who are challenging the dominance of the Big Six incumbents, and providing choice and stronger competition, which benefits consumers.
"We hope you will work with us and Ofgem to stop this Big Six stitch-up, and pledge to help the millions of households who Ofgem seem set to ignore."