Lisa Marie Keen, 37, of Miletree Court, was sentenced to three years imprisonment after admitting manslaughter at Luton Crown Court. That was later reduced to two years on appeal.

On Thursday judges in the Court of Appeal declared her conviction "unsafe" after being told that the House of Lords had made the same ruling in a similar case.

There will be no retrial, and Miss Keen will be freed from prison immediately.

Mr Justice David Clarke, sitting with Lord Justice Hughes and Mr Justice Blair, said Miss Keen was charged after Darren Lebek died at her flat in Clarence Road, Leighton Buzzard in September 2005.

The 35-year-old had injected himself with heroin, and died from a mixture of that drug and alcohol.

Miss Keen helped prepare the fatal dose, although Mr Lebek pushed down the plunger on the syringe.

Lawyers for Miss Keen, who has mental health problems, successfully argued that she had shown a "lack of criminality" and had "wrongly pleaded guilty".

Mr Justice Clarke said that, as the law now stands, those who take heroin "freely and voluntarily" and "self-administer" are responsible for their own actions, and people who take drugs with them cannot be brought before the courts for manslaughter.

The court heard that an ambulance was called to Clarence Road on September 25, 2006, and Mr Lebek was found dead at the scene.

Miss Keen had pleaded guilty on the basis that Mr Lebek had gone to her home and asked if she wanted to go and buy heroin, and they went to a phone box to ring a dealer, before returning to Clarence Road to prepare the drug.