Mrs Clark (54), MP for Peterborough between 1997 and 2005, had been convicted of using threatening words and behaviour to bar staff while drinking at the Great Northern Hotel, in Station Approach, Peterborough, on June 15, 2008.

District Judge Nicholas Leigh-Smith, sitting at Peterborough Magistrates' Court in March this year, said Mrs Clark, of Huntley Grove, Eastfield, Peterborough, was "out of control" when he handed her a two-year conditional discharge.

However, at an appeal hearing at Cambridge Crown Court yesterday, Judge Anthony Bate said although Mrs Clark had used "unparliamentary language" and her behaviour was "shameful", he ruled her conduct was not criminal and overturned the conviction.

Video footage of the incident, taken from a barmaid's mobile phone, later appeared on the website YouTube.

The footage, a key piece of prosecution evidence, showed Mrs Clark shouting "Why have I been treated like this? Why have I been treated like a **** in here? I'm being humiliated," before breaking down in tears.

Judge Bate, sitting alongside two magistrates, cast doubt on the reliability of the evidence given by bar staff, and was concerned about how the video footage had got onto Youtube.

He said the language Mrs Clark used may have caused "temporary alarm, surprise and possible upset" but would not have caused "any reasonable bystander...real emotional disturbance or upset."

Judge Bate added: "The incident was - which Mrs Clark would be the first to acknowledge - shameful and unfortunate,

"She used thoroughly unparliamentary language."

At the trial earlier this year, Peterborough Magistrates' Court heard how Mrs Clark had been drinking in the hotel with husband Alan and another friend for most of the afternoon.

The court was told that Mrs Clark had lost her temper with bar staff when they refused to serve her more alcohol.

Prosecutors said she swore at staff and customers in the bar, allegations denied by Mrs Clark.

Former MP an original 'Blair babe'

Helen Clark was elected as MP for Peterborough on May 2 1997, representing the Labour Party.

She took over from Brian Mawhinney. She lost her seat on May 5, 2005 to current Conservative MP Stewart Jackson.

She was known as one of the original "Blair babes", the group of 101 women Labour MPs elected in 1997.

She had unsuccessfully stood as the Labour candidate for Faversham in Kent in 1992

Early in her parliamentary career she was known for her loyalty to Prime Minister Tony Blair, but spoke out against the Iraq war.