A report from the National Audit Office for the Auditor General for Wales has warned that the final costs of the National Assembly for Wales buildings could be as much as 123 per cent over the original estimates.

The report criticises inadequate monitoring of expenditure in the past and the lack of a fixed budget for the project. The report goes on to criticise arrangements which give the architects, the Richard Rogers partnership, a fee that goes up with the cost of the building and recommended that the assembly should employ its own quantity surveyor to monitor costs, rather than rely on one already employed by the architects.

The development of the assembly buildings is being carried out in two phases. The first is the conversion of Crickhowell House, Cardiff Bay, to be the assembly's temporary home while the permanent home, also in Cardiff Bay, is created.

The costs of converting Crickhowell House have risen from an estimated £5 million to a likely £11.5 million due to extra requirements from politicians. The cost of the new building has risen from the original estimate of £12 million to an expected cost of £23 million.

Officials at the assembly declined to comment before the report was considered by the Audit Committee. However, the former Welsh secretary Alun Michael defended the decisions that were taken, saying: "I am confident that it was the right decision to house the key elements of the assembly - the cabinet, assembly members and level officials - at Crickhowell House." He also backed the calls for effective financial controls on the new building.

Nick Bourne, Welsh Conservative leader, condemned the other parties which had supported the new buildings "for failing to listen to the people of Wales" and confirmed his party's continued opposition to the development plans. Plaid Cymru leader Ieuan Wyn Jones criticised Labour's poor management of the project and said "they should apologise to the people of Wales for their lack of diligence." The Audit Committee of the assembly will take evidence on the report on November 9 from the permanent secretary at the assembly and the principal finance officer among others.