Nisa-Today's has rejected claims by its former non-executive chairman, Edwin Booth, that its buying ­function fails to deliver competitive terms for its members.

Booth, who sensationally quit his post at the group last week, admitted in a submission to the Competition Commission's groceries inquiry that Nisa's buying strength had been severely damaged by the loss of several key members in recent years.

The letter to the inquiry was written in June 2006, before Booth became non-executive chairman in November.

"The acquisition of Jackson's and Bells stores by Sainsbury's, combined with Musgrave's acquisition of Budgens and Londis, effectively led to the buying group function of Nisa-Today's, of which Booths was a member, disbanding," wrote Booth, who stepped down after revealing that Booths would stop sourcing its chilled products from Nisa as of September.

But Nisa-Today's chief executive, Neil Turton, claimed the combination of members' buying power was still central to its business. "Nisa-Today's began as a buying group but has evolved into a buying and distribution group to meet the needs of its members and the ever-changing marketplace," he said.

"Buying and distribution - which represents a £1bn turnover - are the core aspects of Nisa-Today's business and this, combined with ensuring the success of thousands of independent retailers in its membership, is its purpose."

In Booths' submission to the groceries inquiry, Booth claimed that the reduction of buying power had adversely affected Booths' ability to compete with the major multiples.

He said that with only a 0.3% share of the UK grocery market, Booths would often be one of the first in the industry to receive cost price increases from large suppliers.

"Booths had used the buying group to help equalise its buying power with the large fmcg suppliers," he wrote.

"Now, without the support of the buying group it has meant a significant shift in negotiating strength away from the Booths buying team."