A number of British Gas smart meter customers have been unknowingly switched from their cheap tariff of choice to an expensive alternative in an admin error, MoneySavingExpert.com has discovered.

The energy giant didn't notice its mistake, leaving anxious customers to query their abnormally large bills – and in some cases it even tried to hike their direct debits to cover the extra cost.

A dozen customers contacted us to complain about the problem, all of whom joined British Gas on MSE-exclusive cheap tariffs as part of our collective energy switches. 11 had joined in the February 2016 switch and one joined in December '15. It's very possible it affects other British Gas customers too as when we asked, the company refused to rule it out.

MSE has found that in some cases British Gas initially failed to refund customers or put them back on the correct tariff when they first reported the error, forcing them to make repeated complaints.

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What happened?

The customers we spoke to had opted for a British Gas smart meter a few months after switching as part of a collective – many were installed around May.

Those affected had previously been charged on the correct collective switch tariff, but after the smart meter was installed, British Gas mistakenly moved them to the standard variable tariff – its most expensive pricing structure – without their knowledge or consent.

Some found out by noticing the incorrect tariff on their bill, while others spotted unusually high expenditure by looking at the real-time data on the smart meter.

Keith Waterer realised something was wrong when he received an email saying that his monthly direct debit payments needed to nearly double – from around £63 to £115.

"I thought that was odd as I knew I was already overpaying on what British Gas estimated at the start of the agreement," Mr Waterer says.

Fellow customer David Kingston was also distressed when, on July 21, he received notice that his direct debit was going up by 40% – just weeks after having the smart meter installed.

After having a smart meter installed, Louise Threlfall began to receive phone calls from the British Gas sales team urging her to fix her tariff. The first few times she explained she was already on a fixed tariff, and the conversation ended.

However last week, on receiving yet another call, she questioned the sales team and learned she was now on the standard variable tariff, not the MSE collective tariff.

British Gas customers put on most expensive tariff in smart meter error
British Gas says the problem was caused by a "data entry" error

How has British Gas responded?

The customers we spoke to had mixed fortunes when they first brought the error to British Gas' attention, with the energy supplier even saying there was no evidence that Mr Waterer and fellow collective switcher Jennifer Huxley had been on the February switch.

Ms Huxley was proved right when British Gas trawled through her statements and saw evidence of her former tariff, while Mr Waterer was forced to send the company an old confirmation email which showed he had been part of the collective.

Mr Kingston and Ms Huxley have now been refunded for overpayments and their accounts have reverted to the correct tariff. Mr Waterer's account is in the process of being corrected, while Ms Threlfall said hers had yet to be amended.

British Gas says it will now ensure all four accounts are set up correctly.

I'm with British Gas – how do I know if I'm affected?

The issue appears to only affect a small number of people who have recently had a British Gas smart meter installed.

However if you're worried you can check you're currently on the right tariff very easily – it's stated on your online account (if you're registered for one). You can also phone up British Gas on 0800 048 0202 and ask them to check which tariff you're on – though make sure you have your customer reference number to hand when you do this.

If you find you've been incorrectly put on the standard variable tariff, you may be asked to show proof of your original tariff before British Gas will switch you back. If you're asked for this you could show the confirmation email you received when you first switched.

What does British Gas say?

British Gas has attributed the problem to a "data entry error when the customers concerned switched to smart meters." It says there is "no wider problem" and says the number of people affected is "tiny".

MSE gave British Gas details of four affected customers – David Hare, Louise Threlfall, Keith Waterer and David Kingston – all of whom consented to their information being passed on in this way.

A British Gas spokesperson told us it was now correcting these four accounts and added: "We are confident the error won't ever be repeated."

However the company wasn't able to confirm if anyone else had been affected, even though we'd heard from a further eight people who claim they're victims of this error – it remains to be seen if it is a wider issue.

If you think you've been affected by this error, please get in touch with our news team.