Judge Douglas Field, 63, is said to have attended a leaving party during an extended lunch break.
He then instructed his jury to acquit the 55-year-old defendant, who was accused of raping a minor, after they failed to reach a verdict at the end of the five-day hearing.
However, it is claimed Judge Field forgot the man was facing a further two charges and dismissed the panel before they were able to give a decision.
A complaint has been lodged with the Office for Judicial Complaints by the mother of the alleged victim, aged eight. The watchdog is now investigating the claims.
In documents seen by The Sun newspaper, the complainant said in her opinion the judge was "of a person influenced by alcohol", when he returned to court.
She also described how he sat at his bench with his "head in hands resting his elbows on the table", following the leaving party.
The alleged incident occurred at Swindon Crown Court in Wiltshire during a trial 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. Read More