A council spent £45,700 investigating a complaint that its mayor had failed to register his membership of the Campaign Against Political Correctness.
Mayor of Doncaster Peter Davies was found to have made an accidental error and censured for breaching Doncaster Council's code of conduct.
The council hired a solicitor to investigate the complaint.
The "excessive" cost has been criticised as it emerged the bill for the year-long inquiry ran to £45,716.
Mr Davies said: "I am annoyed that for something that was an innocent omission we have been forced to go through an extremely lengthy, costly and quite frankly inappropriate process.
"It has been a waste of taxpayer's money when it could have been recognised long ago that this was a small and innocent mistake that had no intent."
The mayor registered his interests 10 days after coming into office on 12 June 2009, but failed to note his status as a founder and member of the anti-political correctness group.
Mr Davies said he was new into office at the time and had not been fully briefed on what should be included in the Declaration of Trust.
He said as soon as he was informed of the mistake he rectified it immediately.
The complaint was made on 9 September 2009 and passed on to the council's monitoring officer, who appointed a solicitor, the following month.
The solicitor published a final report in May 2010, and the law firm represented the council at the hearing in October 2010.
The investigation found Mr Davies' omission was accidental, rather than deliberate, and he received a written censure - the lowest form of sanction.
The council said the complaint "raised some particularly complex points of law surrounding equalities legislation which the investigating officer had to consider and make a finding on, leading to the investigation being a lengthy one".
Robin Symonds, regional organiser for Unison, said: "Obviously it's a huge amount of money at a time when the council is cutting services and jobs.
"But there is a cost in ensuring that elected politicians abide by the rules."
Charlotte Linacre, of the Taxpayers Alliance, said: "Transparency is key, but the amount of money spent on this one investigation does seem excessive."