Although it has been going on for years, the issue drew widespread attention after the trial of Rosemary West in 1996.

It is estimated up to 19 witnesses received payments from the media for their stories after the "Cromwell Street" trial. By their nature, these payments would have been agreed before the trial's conclusion.

With money at stake, critics argue that witnesses may deliberately embellish their evidence to match up with details they gave to the newspaper.

Max Clifford: Negotiatied the witness payment in the Gary Glitter case

Defenders of the principle claim many of the witnesses would not have come forward but for the cash enticement.

In the case of the Gary Glitter trial, the prosecution's chief witness struck a deal with the News of the World newspaper through publicist Max Clifford.

She went to the paper with allegations that she had been seduced by the singer at the age of 14, and received �10,000 for her story, the court was told. The paper also agreed to put her on a "win bonus" of �25,000 if Glitter were found guilty in court on charges of indecent assault.

Now that Mr Glitter has been cleared of the assault charges, the conduct of the newspaper is likely to come under scrutiny. The Press Complaints Commission, the newspapers' self-governing body on ethical issues, is to investigate.

But the woman's publicist, Max Clifford, said his client stood to receive �25,000 on the basis of an interview with the newspaper, rather than whether a guilty verdict was reached.

'Disappointed at verdict'

He told the BBC he did not think his client had broken the PCC's code of conduct, because she had received her payment before the case came to trial, rather than during it. He added that the decision to pay her a further �25,000 will be between the woman and the newspaper's editor.

Stressing he thought it was difficult to prove whether money would influence people's motives, he said his client was disappointed at the verdict.

"The judge directed the jury" to find the star not guilty, he said, adding that "the ultimate jury is the British public, and what is more important is that the public see Gary Glitter for what he is".

Following the uproar after the West case, the PCC amended its code of practice.

It now states that offers to pay witnesses "must not be made" except where the material concerned is in the public interest and "there is an overriding need to make ... a payment for this to be done".

The proviso is that all sides in court, including the jury, must be told about the offer of payment.

Gary Glitter: Cleared of assault charges

If there is a complaint to the PCC, an editor must be prepared to stand up his belief of "legitimate public interest".

Bob Satchwell, director of the Society of Editors, says the principle of paying for a story which is in the public interest is well established.

"If there's no way that the story will come out any kind of chequebook journalism is justified," he said.

It's a principle practised by broadsheets as well as tabloids, he says.

"It's often the case with whistleblowers that if they go to the press they will lose their job so they need money to compensate for that."

Last year the government signalled it intended to make witness payments an offence. But this has since fallen by the wayside.