The programme, made by Dispatches, to be shown at 8pm claims three-quarters of the marathon's £17.8 million ($27.2 million) turnover is eaten up by salaries and undisclosed costs, including one employee who earns £250,000 per year.

The London Marathon, whose chief executive Nick Bitel is one of Britain's leading sports lawyers, is currently taking legal action over the programme and has threatened to report them to Ofcom, Britain's independent television regulator.

A statement released tonight said: "The London Marathon Limited and the London Marathon Charitable Trust Limited totally deny any allegations of wrongdoing and are surprised and shocked at the lack of evidence presented by the programme’s so-called investigative team to support this desperate attempt to undermine one of the world’s finest sporting events."

The Dispatches investigation focused on the amounts charities pay to the organisers to enter runners in the event, which last year had 35,000 starters and is acknowledged by The Guinness Book of Records as the largest one-day fund-raising event in the world.

It is claimed by London Marathon organisers that since it started in 1981 the race has raised £450 million ($690 million) for hundreds of charities and that figure is expected to raise to half-a-billion pounds after this year's race.

The programme reports that 20,000 runners secure places in the marathon through a ballot, paying £32 ($49) each.

But about 600 charities buy thousands of further places for runners, costing £300 ($458) per place.

These guaranteed places are "hugely over-subscribed" so the London Marathon offers charities without them the chance to buy packages that include adverts in magazines and on websites, the documentary claims.

The cost of getting hold of places as part of these advertising packages can be more than £1,000 ($1,528) each, the programme claims.

London Marathon officials are angry that the company that made the programme - Blakeway - refused to meet them to discuss the allegations.

The London Marathon statement said: "We have repeatedly offered to meet the programme makers - an offer which has been repeatedly rejected.

"They have also refused to allow us on to the programme to say what we wanted to say unedited.
 
"We have nothing to hide and all our financial activities, whether it is the actual organisation of the London Marathon, the distribution of grants from the London Marathon Charitable Trust or our relationships with charities and runners, are transparent and subject to intense scrutiny by independent organisations and approved by them.

"In attacking the money that the London Marathon Limited gives to its charity the programme displays breathtaking ignorance about the way charities trade to raise funds. 

"Virtually every major charity in Britain has a separate trading company. 

"Just like them the whole of the profit of our trading company goes to charity but only after the costs of putting on the event which include costs such as the expense of toilets, barriers, venue hire, staff, rent and all of the other costs of every year staging five world-class events, and putting on Britain’s largest consumer fitness show.

"The programme has also conveniently but not unexpectedly ignored all the positive responses it has received from the various charities, councils and others it has contacted during its so-called investigation."

The London Marathon, which this year is due to take place on April 25 when it will be sponsored by Richard Branson's Virgin Money for the first time, has been supported by the charity sector.

Jo Dyson, the chair of the Event Managers Forum and Institute of Fundraising Special Interest Group, said, "The London Marathon is one of the most important days in the fund-raising calendar for thousands of charities. 

"We recognise that the London Marathon can never satisfy all of the demands from charities and the public for places and believe that in balancing the competing demands it has made the distribution of places as fair as can be in the circumstances."

The last occasion that Dispatches investigated the London Marathon in March 1991 they ended up losing a high-profile High Court libel case and having to pay £1.1 million ($1.6 million) in damages and costs to Chris Brasher and John Disley, the founders of the race.

They sued Dispatches after the programme alleged that it used the event to enrich themselves.
 
David Bedford, the race director of the London Marathon, said: "Perhaps we should not be surprised that a programme that has lied about the London Marathon previously now broadcasts a toxic mixture of opinions and half-truths masquerading as facts."

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