Competition minister Melanie Johnson said the deal had to be subjected to a competition enquiry because Johnston's control of certain local newspaper markets will rise substantially after the deal.
In Northampton, cited as one example, Johnston will, if the deal goes through, control 96% of the market.
Such high concentration of ownership is of considerable concern to advertisers, which are said to be concerned that it would allow Johnston to hike up advertising rates.
The proposed deal involves a £16.1m offer from Johnston for titles including the Peterborough Herald & Post, the Derby Trader and the Northampton Herald & Post.
Johnston Press is also one of three publishers trying to buy Regional Independent Media, the regional newspaper business and publisher of the Yorkshire Post, which was set up when venture capitalist Candover bought the newspaper publishing assets of United Business Media.
The DTI has given the three bidders -- Johnston, Gannett and Guardian Media Group -- another year to put together a bid for the company since its last one was rejected by Candover.
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