Martin Lewis, of MoneySavingExpert.com, said there was a “stark” difference in how providers have handled the mass transfers.

“These providers must up their game, and Ofgem must do more to ensure those caught by its safety net don’t end up trapped in it,” he said.

“Missing credit balances, billing delays and poor communication would be bad enough in isolation – but in the midst of the explosion in consumer energy prices, we mustn’t underestimate the impact of these errors on people’s financial and emotional wellbeing, many of whom are already feeling scared and vulnerable.”

A spokesman at Ofgem said that many of the delays in recovering credit balances were down to data not being passed on to suppliers of last resort. He said: “We have been working with suppliers and administrators to resolve these issues and we expect most refunds of credit balances which have been affected by this issue to be completed soon.”

But Mr Lewis warned that this alone was not the issue. “The consistency in poor feedback for British Gas, Shell Energy and EDF, regardless of which firm’s customers they were taking over, suggests it isn’t just that,” he said.

Energy prices have continued to soar in recent weeks. Suppliers hiked the cost of their fixed-price deals by more than £100 last week after the war in Eastern Europe sparked fears over wholesale gas prices.

Many more families could be forced to change suppliers. More than half of Britain’s remaining small energy suppliers are at imminent risk of collapse, as wholesale prices remain high, according to accountants Price Bailey.

British Gas failed to comment when approached by this newspaper.

The company has taken over the energy supply of more than 700,000 customers from eight failed suppliers.

A spokesman at British Gas said the group had taken on the largest number of customers in the industry and the process of data sharing with failed suppliers had led to the delays.

He said: “We appreciate this is unsettling for customers but the majority of credit balances have now either been applied or will be shortly.”