Last July the OFT launched an investigation into whether the register - dubbed 'a nepotistic old boys' network' by its critics - was unfairly restricting entry for conservation architects.
It was feared that accreditation, which is demanded by an increasing number of government organisations, was difficult for smaller practices to obtain ( Accredited Conservation list faces fair trading probe).
Without this approval non-AABC members cannot receive English Heritage's grant-aid handouts - including those offered in partnership with the Heritage Lottery Fund as repair grants for places of worship.
However, the OFT has rejected the complaint, saying that the register does not breach either the Competition Act 1998 or European legislation.
In support of this, chiefs at the RIBA have pointed to new figures which they say show that 'in spite of the concerns raised by the complaint, the register has continued to grow to more than 300 architect members'.
Former RIBA president George Ferguson said: 'It must now be clear to all that accreditation is legal and decent and is here to stay, giving architects the opportunity of continuing to lead the field in historic-building conservation work.
'We now have a responsibility to develop our understanding and skills, through education and experience, to maintain our professional excellence in this field.'
It is unclear whether a series of proposed changes to the register, revealed in the AJ in December ( RIBA bows to pressure and plans reforms for conservation register), played any part in influencing the OFT decision.