A Freedom of Information request from not-for-profit energy supplier Ebico revealed the £5 million figure, which includes £440,000 spent on commissioning research and external consultants.
Ebico chief executive Phil Levermore claimed the cost to suppliers will also “run into the millions”.
Other costs include £12,100 spent on food and travel expenses and a further £10,900 on event and hearing expenses. The request also revealed the number of full-time staff members working on the investigation rose to 28 people by the end of March this year.
Levermore said: “The CMA investigation has been a complete waste of time and public money. Our FOI request shows it cost £5 million – a huge cost to the public purse when there is very little to show for it.
“The cost to energy suppliers themselves will run into the millions as well, and consumers will ultimately bear the brunt of this in their energy bills.”
Levermore also claimed that the money spent on the investigation could “have been put to better use elsewhere” on issues such as fuel poverty.
“That amount of money could lift 1,100 homes out of fuel poverty and provide materials to insulate 28,000 homes,” he added.