SCOTLAND'S ethics watchdog has been accused by the professional architects' body of bungling the inquiry which cleared Glasgow City Council leader Gordon Matheson of misconduct in the George Square redesign fiasco.

Stuart Allan, the Commissioner for Ethical Standards in Public Life, is being urged to reopen his investigation by the Royal Incorporation of Architects in Scotland (RIAS) and the whistleblower who first complained about the Labour boss.

They say the commissioner failed to interview key witnesses, including the competition judges, failed to cross-check key evidence, included factual errors in the final report, and was too ready to take Matheson's disputed evidence at face value.

RIAS, which ran the George Square design competition, last week wrote to Allan accusing him of abandoning his duty to find the truth behind the council's decision in January to drop the £15 million revamp of the square.

That decision - announced by Matheson moments after the design he wanted to win was rejected by his fellow competition judges - cost taxpayers £100,000 and the competing architectural firms £200,000.

It led to accusations Matheson had "thrown his toys out the pram", and prompted a complaint from the RIAS that he had breached the councillors' code of conduct by showing improper bias in a legally binding European Union procurement process. Matheson was also accused by whistleblower Kerr Robertson, the council's former lead architect, of trying to fix the contest through staff coercion.

The Labour leader denied any wrongdoing. After a six-month investigation, the Public Standards Commission (PSC) cleared Matheson of both complaints - despite recording that Robertson and another council official had corroborated the coercion claim.

In its letter to the commissioner last week, the RIAS said: "Our conclusions are that the report lacks details, lacks investigation and that the conclusions are therefore of doubtful validity. It appears there has been a simple acceptance of what was said by Councillor Matheson without any consideration of issues of credibility and reliability."

In a separate letter, Robertson called the PSC report "fundamentally flawed", "superficial" and a "lamentable failure". He highlighted a reference to a council meeting which the PSC said had agreed to axe the revamp before judging had finished. In fact, it took place after the judging.

Robertson also accused the PSC of quibbling over process while "missing the point that there was an overt attempt to rig the contest".

Neil Baxter, RIAS secretary, said the organisation had now asked the Scottish Public Services Ombudsman to look at the handling of the Matheson complaint. Baxter said: "We are appalled at the extraordinary poor standard of this report … The Standards Commissioner is not up to standard."

Graeme Hendry, leader of the SNP opposition on Glasgow City Council, said: "The information provided by the RIAS and Mr Robertson clearly shows significant gaps and fundamental failings in the approach taken by the Standards Commissioner."

The PSC said it was giving the RIAS letter "full consideration".

A Glasgow City Council spokesman said of the RIAS: "This is desperate stuff … We work exceptionally closely with the SPSO and will give any support they ask for."