A judge has denied being "influenced by alcohol" during a trial.
The Judicial Communications Office said Judge Douglas Field, 63, "emphatically denies" allegations that he was under the influence when he instructed jurors to acquit a 55-year-old defendant accused of raping a minor after they failed to reach a verdict.
The mother of the alleged victim, aged eight, wrote to the Office for Judicial Complaints (OJC) claiming the judge attended a leaving party during an extended lunch break.
She also claimed the judge forgot the man was facing a further two charges and dismissed the jury before it was able to give verdicts on the charges.
A spokesman for the Judicial Communications Office said: "As reported in today's Sun the Office for Judicial Complaints is investigating a complaint into His Honour Judge Field's conduct of a case last year.
"The judge emphatically denies all the allegations and has made a formal complaint to the Press Complaints Commission."
The complaint concerns a trial at Swindon Crown Court in Wiltshire in June last year.
The defendant was accused of raping a girl under the age of 13 as well as two charges of inciting a child to engage in sexual activity. The Crown Prosecution Service sought a retrial last November on the two outstanding charges, but jurors failed to reach a verdict and the defendant was formally cleared.
An OJC spokesman said: "A complaint has been received about His Honour Judge Field in relation to a trial at Swindon Crown Court in June 2010. This is being investigated. It would be inappropriate to comment further at this stage."
Judge Field refused to comment on the allegations when he was approached at his home in Warminster by the newspaper.