“As you can see from our signatures below, the idea has substantial cross-party support,” the MPs wrote.
“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-6 stitch-up, and pledge to help the millions of households who Ofgem seem set to ignore.”
Among the signatories are the former ministers Iain Duncan Smith, Andrew Mitchell and Caroline Spelman.
The letter calls on Mrs May to introduce a relative price cap, which has found favour with some Tory MPs idealogically opposed to interventions in free markets.
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.