Retailer Woolworths' purchase of book wholesaler Bertram has been referred to regulators amid concerns it will stifle competition in the sector.
The worry is that non-blockbuster books will not get on shelves
The Office of Fair Trading (OFT) has asked the Competition Commission to look at the deal as the new firm would control more than 40% of the market.
Its main rival, Gardner, has a similar share and there are fears independent book firms will have too little choice.
Woolworths agreed to buy Bertram for £29m at the start of this year.
The retailer, whose shares fell by 1.6% in London on the news, said it would cooperate with the inquiry.
A decision on the deal is expected by 14 September.
"UK consumers spend some £360m a year in independent bookstores," said Vincent Smith, senior director for competition at the OFT.
Mr Smith explained that a key reason for this demand was "because these retailers stock a range of 'backlist' and 'mid-list' titles and offer specialist advice and service which they value".
He added that retailers had voiced concerns over a tie up between Woolworths and Bertram.
Woolworths last week reported a 74% drop in full-year profits, blaming a decline in entertainment and confectionery sales.