Nearly half of vulnerable customers are put off from switching energy suppliers because they are afraid of losing their Warm Home Discount (WHD) benefit, a survey has found.

The research, carried out for supplier First Utility, follows a cross-party call by MPs last week on the government to review smaller suppliers’ exemption from the scheme.

Under current rules energy suppliers with fewer than 250,000 customers are exempt from both WHD and the Energy Company Obligation (Eco) energy efficiency improvement scheme.

According to First Utility’s survey of WHD recipients, 42 per cent said they would not switch suppliers because they are afraid of losing their entitlement to the £140 energy bill rebate for low income households.

The same survey shows 63 per cent of WHD recipients would be more likely to switch if all suppliers offered the WHD.

Only 17 per cent of households in receipt of the WHD would consider switching to a supplier that didn’t offer the WHD, a figure which drops to 8 per cent for those over 55 years old.

A total of 25 Labour, Conservative and SNP MPs wrote to prime minister Theresa May and the business secretary Greg Clark, calling for the ending of the exemption so all vulnerable customers received the same guaranteed support irrespective of their energy supplier.

They said in the letter the exemption should be reviewed given the number of customers with suppliers, which are exempt from the two schemes, has trebled to one in 12 of the energy supply market in the past three years.

Citizens Advice has also called on the government to review the exemption.

Ed Kamm, chief commercial officer of First Utility, said: “Our new research clearly highlights that people aren’t switching to cheaper deals for fear of missing out on the Warm Home Discount.

“The government has to change this outmoded scheme so all suppliers contribute and customers feel confident about changing supplier to save money. With Citizens Advice and a swathe of MPs now calling for the same thing, surely there is enough evidence to ensure all suppliers participate in the scheme regardless of size.”

A total of 16 companies are currently participating in the WHD, including council owned Bristol Energy, which is doing so on a voluntary basis although it has fewer than 250,000 customers.

First Utility was acquired by Shell earlier this year.