The proposed deal will lead to a ‘private cartel’ and needless price hikes, according to the union.

The GMB has asked business secretary Greg Clark to block the proposed merger of SSE and Npower.

In a letter to Clark, the trade union warned it could turn the Big Six largest energy providers into the “Big Five” and create “a chokehold that is bad for customers, workers and the wider economy”.

It said if the Competition and Markets Authority (CMA) doesn’t initiate an investigation, Clark should use his powers under the Enterprise Act 2002 to block the merger on the grounds of public interest.

Justin Bowden, GMB’s national secretary for energy, said: “The merger between SSE and Npower is a test of that duty [to keep energy secure, cheap and clean] and until there is a settled energy policy, we risk a private cartel if the merger is given approval.”

And he urged Clark “to exercise existing powers to prevent yet more needless price hikes that are the penalty for a dysfunctional energy market”.

SSE is the UK’s second biggest energy supplier and Npower the sixth, but together they will have more domestic electricity customers than British Gas, the market leader, and will be second only to British Gas for gas customers.

When contacted, Npower and SSE confirmed they are in discussions with the CMA. Tony Keeling, chief operating officer of retail at SSE, said the merger would “improve competition in the market”, and “ultimately offer better value for customers”.

Last week, the consumer group Citizens Advice warned the merger could have a “dramatic impact” on competition in the energy market.

The group’s director of energy Victoria McGregor told Utility Week the energy market has “long failed many people”, so it is vital any new company resulting from the merger of SSE and Npower is “clear on the value it will offer customers, not just shareholders”