The complaint was officially quashed last week by the investigation committee of the Solicitors Complaints Bureau after Titmuss appealed against a "severe rebuke" against one of its partners, Peter Gold, in 1993.
Graham Brown, who made the complaint five years ago over the handling of a rights issue for Oasis, called the process "convoluted and biased" and vowed to take the matter up with the Legal Services Ombudsman.
His attack comes on the heels of a recent report from the Ombudsman that severely criticised the handling of complaints and warned the Law Society it could face outside regulation unless it put its house in order.
Mr Brown said the Bureau failed to show him the original case officer's report, despite an assurance that he would see all evidence in line with the rules. When Titmuss appealed, it then refused to release anything to Mr Brown until he signed a non-disclosure agreement.
As that would have prevented him from disclosing even the outcome of the case, he refused. As a result, and in breach of the rules, he was denied access to the Titmuss appeal documents.
A Bureau spokesman declined to comment on the case.Reuse content