The competition watchdog is investigating the anticipated joint venture between South Staffordshire Water and South West Water.
The two companies plan to merge to provide retail water and sewerage services to the non-household sector.
Around 1.2 million businesses and public bodies in England will be able to choose their retail supplier, which is currently only available to the largest users. The English water market will open in April this year.
The Competition and Markets Authority (CMA) will look into whether the joint venture would result in “a substantial lessening of competition within any market or markets in the United Kingdom for goods or services”.
It is inviting views until 16th February.
Last November the CMA also approved the purchase of Bournemouth Water by South West Water.