The Office of Fair Trading is seeking comments on the merger between The Co-operative Travel and Thomas Cook.
The OFT is considering asking for jurisdiction from the European Commission to look at the merger under the UK merger control regime.
The retail merger, announced last month, would create the largest high-street agency chain in the UK, with 1,204 shops and a customer base of 4.3m. It is expected to generate savings of £35m a year.
Concerns about the merger are centred on job losses, shop closures and the health of the independent sector, with third-party operators facing the loss of Co-operative Travel as a distribution channel.
Those who wish to comment should send written representations about any implications the merger may have for competition in the UK by November 19 to:
Office of Fair Trading
2-6 Salisbury Square
London EC4Y 8JX
Fax: 020 7211 8916