It was revealed last week that the selection panel considering shortlisted architects are to recommend that Mecanoo should design the £20 million transformation.

Austin-Smith:Lord/ MVRDV, Hoskins Architects, LDN and Richard Murphy Architects were also shortlisted following a design competition which attracted global interest and over 70 initial expressions of interest.

However, with councillors set to make their decision on the project this evening, the RIAS has said it could only concur with unsuccessful practices that “expressed their disappointment at the council’s variance from the procedure set out in the bid documents”.

Scoring criteria to assess the shortlisted entries against the brief, which were developed by RIAS and agreed with council officers, were weighted 80% of the scoring criteria attributable to architectural quality and 20% to the fee price. The relatively high ratio of quality to price was agreed “to reflect the importance of the design concept to the council”.

A report recommending the appointment of Mecanoo to the project show that the Dutch practice initially came third in the competition, based on the combined scores for quality of design and proposed fee.

The practice did, however, comfortably top the shortlist based on design alone.

Council officers felt the low marks on fees had disproportionally weighed against the firm, and that Mecanoo’s design concept “is the only one which, on the basis of the estimated construction cost, can be delivered for the available project budget”.

The Incorporation said last night that it had taken an “unusual step” in publishing its correspondence with the council.

In a letter to council leader Ian Campbell, RIAS president Stewart Henderson said the organisation agreed with shortlisted practices disappointed by the competition outcome.

He said: “We understand from council papers for this Wednesday’s meeting that the officers of the council are proposing to recommend Mecanoo to take forward the Perth City Hall project.

“You will appreciate this is contrary to the RIAS report following the recent quality/prize driven process for which we were appointed.

“Unsuccessful practices have expressed their disappointment at the variance from the procedure set out in the bid documents. We can only concur.”