The ACS is calling on the competition authorities to closely scrutinise Tesco's acquisition of the CTN and c-store business of independent retailer Mills Group.

The Tesco-owned c-store chain One Stop announced this week that it was buying the 77-store North East-based chain for an undisclosed sum.

It said the deal would take its store count to 598 stores across England and Wales, giving it a 1.2% share of the convenience market based on IGD figures.

However, the ACS pointed out that it would also propel Tesco's convenience estate to almost 1,800 stores or 27% of the multiple c-store market given that it already has 1,183 Tesco Express stores.

"The competition authorities have repeatedly failed to grasp the implications of the continuing growth of Tesco's shadow brand," said ACS CEO James Lowman.

"The OFT allowed the original acquisition by Tesco of over 1,000 T&S stores without adequate scrutiny in 2002 and the Competition Commission failed to address the issue. They must not make the same mistake again."

The deal is subject to regulatory approval but is expected to be completed by the spring. One Stop CEO David Turner said the deal would allow it to refurbish the stores and lower prices.

Mills Group, which is number 22 in The Grocer Top 50, started out as a CTN operator but has diversified into convenience and supermarkets in recent years.

CEO Nigel Mills said that the deal did not include the company's six ­supermarkets, which it ­acquired from The Co-operative Group last year. These would continue under the Nisa Extra banner the symbol fascia supplied by buying group Nisa-Today's.

Nisa-Today's CFO John Schofield added that Mills Group represented 2.3% of Nisa's business around £35m of its £1.42bn turnover.

"The loss of Mills Group c-stores is disappointing, but we are ­confident it will not affect the company's overall health," he said.