SWINDON-based energy supplier Npower is apologising to thousands of its customers after they had their personal details sent out in the post.

The energy giant, which has its HQ on Windmill Business Park, was alerted to the error only after a retired GP shared a letter he received with customers names, addresses and payment amounts on the back of his statement.

The details did not include bank details.

The letters were a mailing for the company’s Feed-In Tariffs scheme for those with solar panels.

Retired GP Dr Tom Harris, from Somerset, told the BBC he received one of the letters over the weekend.

He said: “When I opened it the front page was addressed to me but overleaf were personal details of another customer. And there were another two sheets of A4 with the details of three others.

He said when he contacted Npower “they didn’t seem unduly surprised” and that the company “was aware of other people in the same situation”.

An Npower spokeswoman apologised to its customers and sought to distance itself from the error.

“We’re urgently investigating how this occurred with our fulfilment partner, who sent the mailing on our behalf.

“We apologise for this error, especially to the customers whose information was incorrectly shared – around 5,000 in total.”

The Information Commissioner has been notified of the breach and is making enquiries into how it occurred.

The news comes only two weeks after Npower was given the go-ahead to merge with rival firm Scottish Southern Electric (SSE), the UK's second largest supplier after British Gas, with 11.5 million customers.

In January 2016 the ICO fined SSE £1,000 for mistakenly sending just one customer the name and account number of another customer. The regulator can issue fines up to £500,000 to companies.