The Office of Fair Trading (OFT) is examining C&C Group's purchase of the Tennent's lager brand to see if it is a "merger situation" which could lead to reduced competition in the market.
In August, drinks giant AB InBev sold its businesses in Ireland, Northern Ireland and Scotland - including its Tennent's brands - to C&C Group for £180m.
However a spokesman for C&C Group, which also produces Magners cider, told The Wall Street Journal: "This is a normal course of business for the OFT. The transaction was completed after shareholder approval and Irish Competition Authority approval. The transaction cannot be reversed."
In a statement to the Stock Exchange the OFT said: "Should it be found to be a relevant merger situation, the OFT will further consider whether the creation of that situation may be expected to result in a substantial lessening of competition within any market or markets in the United Kingdom for goods or services that warrants reference to the Competition Commission for investigation and report."
The deadline for submissions is October 19 and can be sent to: Marie Didierlaurent; Office of Fair Trading. Fleetbank House. 2-6 Salisbury Square. London EC4Y 8JX or email: firstname.lastname@example.org or fax: 020 7211 8916.