Like a schoolboy emerging from a spelling test, he proudly brandished a certificate bearing the words "well done".

But Pete Doherty's precious piece of paper proved only that he had passed a drugs test while in prison.

He seemed delighted to have it as he left just 29 days into a 14-week sentence, declaring he wanted to "keep it together and work on my music".

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Clean: Pete Doherty leaves prison clutching his certificate

The 29-year-old singer, who has been repeatedly arrested in possession of crack cocaine and heroin, was finally jailed last month for breaching his probation.

But yet again the law smiled on Doherty by setting him free early.

He emerged from Wormwood Scrubs in West London at 7.10am yesterday in a suit, tie, what appeared to be rosary beads and surprisingly shiny shoes.

Smoking a cigarette, he said he was looking forward to having a drink and seeing his pet cats.

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Free: Pete leaves Wormwood Scrubs prison

Doherty, whose relationship with Kate Moss ended after his drug problem got out of hand, happily chatted to reporters waiting outside the prison.

The certificate shows Pete passed drugs tests

He described prison life as "a lot of gangsters and Radio 4".

Asked if reports that he had taken heroin while inside were true, he replied: "Well, I knew it was going to be a bit rough to start with, with the overcrowding and the medical facilities although they do their best - they are good, they can't really cater for the average junkie..."

As he walked through the prison's exit Doherty said, confusingly: "Thank you Mrs Thatcher for putting me in the company of the most dangerous criminals in the country."

He then cadged a lift to his rented home in Marlborough, Wiltshire.

"Prison was hard. I don't really want to think too much about it now that I'm free," he said later.

"I missed the cats, my home and alcohol.

"I haven't had a drink for ages and it feels good to have a glass of red wine. It was the first thing I did."

Back home: Doherty at the property he is renting in Wiltshire