The minimum wage which is currently set at £5.73 per hour for workers over 22, £4.77 for those aged 18-21, and £3.53 per hour for under-18s.

But campaigning freelance Mark Turner says many media organisations are breaking the rules by using casual staff in unpaid roles.

Now he has presented a 70-page dossier of evidence to the Low Pay Commission about the extent of the practice in a bid to persuade both it and the government to enforce the law.

The dossier contains examples from actual job ads that have been placed which make it clear that the role concerned is unpaid.

One ad for an editorial assistant placed by a leading marketing magazine concluded: "This is a work experience role. Performance will be reviewed after three and six months with a view to offering a full-term contract after that time."

Said Mark: "The regulations dictate that it is not in the gift of any individual to cede the minimum wage. The regulations should be enforced and these people should be paid.

"The widespread failure to pay the minimum wage has a wider effect on the whole of society. Every employer using an unpaid worker is depriving the exchequer of tax and national insurance that should be paid on these wages."

Some prominent commentators and academics have also expressed disquiet about the practice which they claim is turning journalism into a profession for the children of rich parents.

The media blogger Roy Greenslade, who is also a professor of journalism at London City University, said: "It really does seem extraordinary that we have allowed this exploitative system to grow up without questioning whether it is good for journalism.

"It is obviously beneficial for employers, but it must restrict the social mix of people coming into the industry."

Fellow City professor Paul Greengrass added: "We're arranging an industry where most people start by working for nothing, so they will tend to be people whose parents support them."

In a report last year, the Low Pay Commission acknowledged the problem and promised to issue guidance to employers to reduce "non-compliance."

"It has become the norm in some parts of the media to expect prospective newcomers to offer their time for little or no financial reward as the price of entry to that industry," it said.

However although the guidance was issued, Mark Turner said the evidence in his report suggested little had changed since then.

"The evidence would suggest that the effect on employers of this guidance has been limited and that unpaid work is now widespread in a number of popular career choices for young people," he said.