Four trustees of the Tata charities that control the group have accused two fellow trustees of breaking ranks over the long-held opposition to listing Tata Sons, deepening a dispute at the top of the 277-billion-dollar Tata empire.In a letter dated October 5 to trustees Venu Srinivasan and Vijay Singh, Noel Tata, his son Neville Tata, senior lawyer Darius Khambata and veteran executive Bhaskar Bhat said exploring ways to keep Tata Sons unlisted was consistent with positions approved by the seven charitable trusts that own 66 per cent of the company.The rift centres on a September 28 proposal to merge Tata Consulting Engineers and Tata Electronics Systems with Tata Sons. The merger would reduce the holding company's financial character, helping it exit the RBI's upper-layer NBFC classification and avoid a mandatory listing. The RBI rejected Tata Sons' deregistration request on September 11.Srinivasan and Singh have objected to the plan, saying they were not consulted. The Sir Dorabji Tata Trust has since backed Noel Tata's position. Tata Sons' board had voted 4 to 1 on September 17 to proceed with the listing process, with only Noel Tata opposed.