One of Britain's top judges was cleared today of exposing himself to a woman on a train.

Sir Stephen Richards, 56, who sits in the Court of Appeal as Lord Justice Richards, left the City of Westminster Magistrates' court a free man after being cleared of intentionally exposing himself to a fellow commuter twice on trains in south London last year.

A panel of three magistrates, made up of Timothy Workman, the senior district judge for England and Wales and two women lay magistrates, ruled there was no independent evidence to identify him as the guilty man. Mr Workman also criticised the British Transport Police for failing to carry out a thorough investigation.

Sir Stephen was charged under the Sexual Offences Act with two counts of "intentionally exposing his genitals intending that someone would see them and would be caused alarm or distress".

During the trial, the judge said he was astonished when he was arrested by police at Waterloo station in January after being identified by a woman in her twenties as the "very kind-looking" man who twice approached her on crowded trains with his flies undone.

The woman told police that she originally thought the man had mistakenly exposed himself but changed her mind when he came after her a second time a week later. On this occasion, she followed the man when he got off, taking pictures on her mobile phone and trailing him to near the Royal Courts of Justice in The Strand.

The judge's defence rested on mistaken identity and the design of the Calvin Klein underpants that he usually favours. Giving evidence yesterday, Sir Stephen said that he would need to use two hands to expose himself in the manner alleged by his accuser.

He also said it would make no sense for him to expose himself on the train he takes every day between Waterloo and Wimbledon, where he has lived most his life and is well known, not least as the chairman of the governors of King's College School.

"I cannot see how one could do what is alleged to have been done without the very high risk of being seen or detected by other passengers," he told the court. "For me personally the risk would be further and very considerable by the fact that so many people know me that are liable to use that service."

Dismissing the charges today, Mr Workman described Sir Stephen's accuser as an "honest and truthful witness" but said the police had not tried to find any evidence to corroborate the allegations, which dated from October last year, before arresting the judge.

"Had they done so, they would have been able to obtain closed circuit television from the train on October 24 and probably on October 16," he said. "That evidence may well have supported her identification and, equally, may have exonerated Sir Stephen."

"There was no supporting evidence obtained during the investigation," he concluded. "For this reason only, (we) reach the conclusion that the evidence is insufficient to meet the high standards required on identification and we cannot be satisfied beyond a reasonable doubt. The charges are therefore dismissed."

On the steps outside court, Sir Stephen said he and his wife, Lady Richards, were looking forward to resuming their normal lives.

"Throughout the trial I have put my trust in the legal process and I am delighted that it has enabled me to clear my name," he said.

The judge also thanked his family and friends for their support. In the stand, Sir Stephen had testified that he had a happy, fulfilled marriage and was fortunate to have a close and supportive family. "I am a happily married, family man and I cannot perceive deriving any form of gratification to exposing my penis," he told the court.

Sir Stephen, who was not been sitting on trials but has handled paper applications while his case was pending, works at the Court of Appeal and has presided over several high-profile cases, including the complaint brought by the family of Jean Charles de Menezes, the innocent Brazilian who was shot dead in July 2005 by police who mistook him for a suicide bomber.

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