A former porn star who was jailed for two-and-a-half years for stabbing her boyfriend after an argument over missing cocaine wept in the dock as she watched top judges almost double her sentence.

Melissa Walker, 26, was a middle class university graduate with a bright future ahead of her when she became caught up in a world of pornography and cocaine abuse.

In June, she was convicted at Lewes Crown Court of wounding with intent to cause grievous bodily harm after an attack on her then boyfriend, Matthew Quant, at her then home in Denmark Villas, Hove.

Yesterday at London's Criminal Appeal Court she bowed her head and wept as Lord Justice Maurice Kay read out the Appeal Court's verdict, ruling her sentence "unduly lenient" and increasing it to four years and nine months.

The case came about after the Attorney General, Baroness Scotland QC, referred Walker's sentence to the Appeal Court, asking the judges to increase it.

Walker sat quietly in the dock and mouthed the words "I love you" to her parents - watching from the public gallery - as the judge spelled out the details of the offence.

She had been in a volatile relationship, characterised by cocaine and alcohol use, with Mr Quant, a 22-year-old scaffolder, for around six months when the attack took place in November 2005.

After a night of heavy cocaine use at her flat, the pair began arguing when Mr Quant was unable to find their last gram of the drug. Walker, who allegedly had a £1,000-a-week habit, became increasingly agitated.

She hid behind a door in the front room and, when Mr Quant returned from using the toilet, jumped out and plunged an ornamental knife with a partially serrated eight-inch blade into his abdomen.

Then, as he attempted to take it from her, she stabbed again, this time into his chest, causing wounds that were nearly fatal.

She called police and told them she had stabbed him because she had no other choice as he was "hurting her badly".

The case went to trial in November, but the jury could not reach a verdict, and it was not re-started until this summer.

When it was, the jury disbelieved the self-defence story and convicted Walker, who had by then moved back to her home town of Lincoln.

Judge Richard Brown, at the Crown Court, said she had been "up to her neck" in cocaine and alcohol and had shown no remorse, but then handed her the lenient two-and-a-half year sentence, prompting the Attorney General's reference.

Giving the court's judgment, Lord Justice Maurice Kay, who sat with Mr Justice Jack and Mr Justice Simon, said: "This is one of those tragic cases of a young person who originally had every advantage in life.

"She came from a good home, is highly intelligent, was successful at school, obtained a degree at university and had an excellent life before she succumbed to the ravages of cocaine."

He added: "Having regard to the wide bracket of sentences, it is immediately apparent that the sentence passed on this offender was not just lenient, but was unduly lenient.

"Cases under section 18 wounding with intent involving the use of a knife are extremely serious.

"It's appropriate for the court to have regard to all mitigating factors in favour of an offender, even when those mitigating factors are relatively few, as they were here.

"But, ultimately, the task of the court is to meet the circumstances of the offence with punishment which is appropriate to those circumstances and not just to the circumstances of the offender."