As he begins a 16-week sentence at Belmarsh prison this week, Kenneth Cooper may count himself among the most unfortunate of petty criminals.

The small, middle-aged woman must have looked a promising target with her bag wide open; the crush of a bus queue would have seemed the perfect place.

But if his crime was opportunist, this was not the opportunity Cooper may have hoped it would be. His victim was Bridget Prentice, the Justice Minister; an off-duty detective was standing right behind her and, as he fled, Cooper ran headlong into a busload of police officers who just happened to be on a “safer neighbourhood” crime prevention exercise.

With hindsight his modest scheme would rank as one of the most ambitious and foolhardy ploys in the recent annals of petty crime.

Ms Prentice, 55, who is the MP for Lewisham East and a junior minister in the Justice Ministry, had been on her way to attend a citizenship ceremony.

As she boarded a bus in Lewisham town centre she felt a man pushing up against her. “I was just turning round to ask him to stand back when he got off the bus,” she said. “I knew straightaway that something was wrong and looked into my bag to discover my purse had gone.”

“I shouted, ‘He’s nicked my purse,’ and a man farther back in the queue immediately gave chase.” This man, whom Ms Prentice would describe as her “knight in shining armour”, turned out to be a off-duty detective sergeant.

With the young officer hot on his heels, Cooper, 46, ran across the road and on to a pavement — where a team of police officers had just arrived to offer crime prevention advice to the shoppers.

They immediately joined the chase. Cooper ran into a shopping arcade, but the pursuing police alerted security guards and soon Cooper — who may have been envisaging easier targets at the start of the day — found himself up against the combined forces of Lewisham’s finest crime prevention officers and the guardians of the Riverdale shopping centre.

It was an uneven contest. A spokesman for the Metropolitan Police said: “The man was detained and the purse recovered.

“It was fortunate that the thief ran past a busload of police and he was caught so quickly.”

Cooper, from Kennington, South London, came before District Judge Michael Kelly at Greenwich Magistrates’ Court where — wisely, perhaps — he had decided to plead guilty to the theft.

Ms Prentice said that she had been left shaken by this first-hand contact with crime in her constituency. She said: “I am still confident that Lewisham town centre is a safe place. A lot of it was down to me having left my bag wide open.”

Colin Finch, Lewisham Victim Support manager, said that petty crime was common on buses and that the victims were usually elderly women. “The way this incident happened is pretty typical and Bridget, like other victims of crime, shouldn’t blame herself.”

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