LONDON (Reuters) - Co-operative Group (>> Co-Operative Group Ltd) is in exclusive talks to buy Nisa, entering the fray days after larger rival Sainsbury's (>> J Sainsbury) suspended its own bid talks for the wholesale group.

Britain's biggest retailers have set their sights on operators within the fast-growing convenience store sector after Tesco (>> Tesco) agreed to buy Booker (>> Booker Group) for 3.7 billion pounds ($4.8 billion).

Sainsbury's, the country's second largest supermarket, said on Aug. 15 it had suspended bid talks with Nisa until the regulator gave its verdict on the Tesco-Booker deal, leaving the door open to the Co-op.

In a letter to shareholders, Nisa said it had granted the Co-op a period of exclusive access to its books from Wednesday.

"Thereafter, and subject to the results of the due diligence, it is anticipated that the Co-op could be in a position to make a final offer to the members for your consideration," Nisa Chairman Peter Hartley said.

Nisa's members operate almost 2,500 retail stores around the country.

($1 = 0.7735 pounds)

(Reporting by Fanny Potkin)

Stocks treated in this article : Tesco, Booker Group, J Sainsbury, Co-Operative Group Ltd