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 SNP MPs and Labour MPs, threatens to cause trouble for May’s fragile government.

Patricia Gibson, the SNP’s frontbench spokesperson on consumer affairs, is among 25 SNP signatories to a letter to May. Caroline Flint, a former shadow energy secretary, is among the Labour signatories.

The Conservatives promised before the election 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.

Instead, after May’s re-election, business secretary Greg Clark wrote to the energy regulator Ofgem asking it to safeguard “customers on the poorest value tariffs”.

Labour, at the time, accused the government of betraying millions of households.

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 per cent for 3.1 million customers.

In the letter to the PM, which will be delivered to Downing Street after the parliamentary recess, they say: “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.”

It was reported yesterday that some backbenchers are considering tabling an amendment to Philip Hammond’s next Budget, due in November, to force the government’s hand.

Among the signatories to the letter sent to May are former Cabinet ministers Andrew Mitchell and Caroline Spelman.

Earlier this week the head of energy regulator Ofgem said it was up to ministers to bring forward legislation if they want to cap household bills.

Unlike an absolute cap, which would set an overall maximum price, a relative cap would set a maximum mark-up between each energy firm’s best deal and the standard variable tariff paid by most customers.

As a result, if customers failed to switch tariffs at the end of their existing deals, their bills could only increase by a limited amount.

Data published from the regulator last month shows that British Gas, EDF, E.ON, Npower and Scottish Power have increased the profit margin they make on those paying bills from 3.7 per cent on average in the 2015-16 financial year to 4 per cent in 2016-17.

Ofgem also published figures revealing that, while the costs faced by suppliers in May were up 15 per cent on the same time the year before, they had fallen 6 per cent between February and May.

Figures released by Ofgem show that 65 per cent of people are still on a standard variable tariff (SVT), down fractionally from the 66 per cent when the data was first published last December.

A landmark investigation by the Competition and Markets Authority last year concluded many households on SVT were paying too much for their energy.

It calculated consumers were overpaying by up to £1.2 billion a year.