UK energy giant SSE and German-owned Npower revealed an agreement to form a new UK gas and electricity business last month.
In a letter to the Competition and Markets Authority (CMA), Reeves said the announcement was "concerning" for the market.
"The proposed merger between SSE and Npower risks damaging the development of a more competitive energy market, reducing consumer choice, and threatening to be a bad deal for energy consumers," said Rachel Reeves, chair of the select committee.
"The CMA needs to look at the potential impacts of this merger and launch a full investigation if there is any risk to competition within the energy market."
The move comes as the government works to tackle "rip off" energy prices by legislating a temporary price cap on energy tariffs.
A spokesperson for SSE said: “The merger will improve competition by offering customers a completely new model combining the resources of established players with the agility and innovation of an independent supplier – turning 60 competitors into 59, not five, and ultimately offering better value for customers.
"This is, naturally, subject to the appropriate regulatory approvals and we will engage openly with the CMA, the Beis select committee and any other interested parties as the process goes on.”
An Innogy spokesperson said: “Npower and SSE are currently in contact with the CMA and will notify the transaction formally to the CMA once these preliminary discussions are complete. The transaction will create a new, strong and independent British retail energy supplier, which will provide a better deal for consumers and help deliver the government’s plans for more competition in the market.”