Lawmakers said the deal could reduce competition and affect consumers as together with Centrica's British Gas, the combined company would control more than half of the UK market.
SSE and Innogy said last month they would merge and list their British retail units to better compete with smaller rivals and reap badly-need synergies in a market with thin margins.
The deal, which would give SSE a stake of about two thirds in the combined entity, is expected to be completed by the first quarter of 2019 at the latest.
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A cross-party parliamentary committee called on the Competition and Markets Authority (CMA) to look into a merger that would reduce Britain's Big Six energy companies to five.
"(We) would welcome the confirmation of the CMA that it will look in detail at the potential impacts that this merger could have on the operation of the domestic energy market, and undertake a full investigation if there is any risk of a lessening of competition within the sector," Rachel Reeves, chair of the cross-parliamentary Business, Energy and Industrial Strategy (BEIS) Committee, wrote in a letter to the CMA.
(Reporting by Nina Chestney; editing by Alexander Smith)
