Co-operative Energy, First Utility, Ovo Energy and GB Energy are the “trickiest” big energy suppliers to leave, according to a survey by Which?

At least 6 per cent of customers switching away from them said they found it difficult or very difficult.

The switch being “slow” was the most common reason (23 per cent) customers gave as to why they struggled with the process.

Overall, the trickiest transfer was from EDF Energy to Npower, with a fifth of customers rating it difficult or very difficult, the survey found.

British Gas or Eon to Npower were the next trickiest switches with eight and nine per cent respectively finding it difficult to move between firms.

Other reasons customers said it was difficult to switch energy supplier included: difficulty getting a refund (15 per cent), difficulty paying previous supplier (15 per cent), poor communication from new firm (9 per cent), inadequate customer service from new supplier (8 per cent), problems setting up an online account (5 per cent) and problems paying the new firm (4 per cent).

An Ovo Energy spokesperson, said: “We would encourage everyone to shop around for the best deal on the market for them. We’re reviewing the findings of the Which? survey and will continue to keep working out ways to make life easier and better for all of our energy customers.”

Meanwhile Sainsbury’s Energy and SSE were found to be the easiest to leave with 92 per cent of respondents rating them as easy or very easy.

Overall switching from British Gas to First Utility was rated the easiest, with 97 per cent saying it was very easy or easy.

A spokesperson from British Gas, said: “We are committed to making switching faster and easier for our customers.

“We are signed up to the Energy Switch Guarantee – a set of principles around what customers can expect when switching supplier. The rollout of smart meters will help enable quicker and easier switching. British Gas is leading the rollout of this technology.”

Figures from Energy UK revealed 5.5 million people switched electricity supplier last year. Around 660,000 customers switched in February this year, which was a 60 per cent increase from the same month in 2017.

An Energy UK spokesperson, said: “Nine in ten energy switchers say they are happy with the process and with the Energy Switch Guarantee, which now covers over 90 per cent of the market, it means consumers can be assured that switching will be simple, speedy and safe.”

The online survey of 8,397 UK energy customers carried out in September 2017 by Which? looked at 31 gas and electricity suppliers and all six Northern Ireland energy firms with at least 30 respondents who had switched away from each company.