Nearly half the runners are sponsored

Some charities and tax experts say runners pledging to raise a minimum amount could see 17.5% going into the government's coffers.

But the Treasury said there has been no change in the VAT rules and that there would be no clampdown on charities.

The organisers of the marathon, itself a charity, said changes in tax enforcement would cost them �700,000.

The Treasury says if a charity is simply charging runners to enter the race, and effectively acting as a middleman, then that constitutes a sale on which VAT is chargeable.

But a spokesman said reports of VAT charging were misleading and that every effort is being made to minimise charities' tax burden, although some say they face new costs.

The Treasury said in a statement: "There has been no change in the VAT rules, there is no question of a clampdown.

To use this to punish charities is both wrong and inappropriate Nick Bitel London Marathon chief executive

"And the idea 17.5% of the money raised by sponsored runners in the marathon goes to the Treasury is just nonsense.

"Customs are working with the charities sector to ensure charities pay the minimum amount of tax possible on the funds raised by sponsored runners.

"The government is firmly committed to the charitable sector and provides �2.2bn worth of tax reliefs and special provisions to charities each year."

The Treasury said that charities could avoid paying tax by changing the wording on runners' pledges.

Nick Bitel, the Flora London Marathon chief executive, told BBC News Online that Treasury suggestions that VAT would only be levied on the part of a pledge that paid for the �250 race fee was not what charities were experiencing in practice.

"I've spoken to a number of charities and certainly they say what the Treasury says to do has not been the practice of Customs and Excise so far.

"They are saying that if the charity says you must raise X amount then that they say is a sale.

"To use this to punish charities is both wrong and inappropriate. They are also having a go at the marathon itself."

Nearly half of the 33,000 runners in Britain's largest sporting event will be raising money for a charity.

Chartered accountants Chantrey Vellacott DFK's charity tax expert Peter Ladanyi said the reported plan to charge VAT was "completely wrong".

"If it is taxable the government should refund it."

Gordon Brown must act urgently to ensure charitable donations remain exempt from VAT Simon Hughes MP

Simon Hughes MP, the Liberal Democrat candidate for London mayor, told the Times newspaper: "Gordon Brown must act urgently to ensure charitable donations remain exempt from VAT."

Peter Reynolds, director of fundraising for Breakthrough, told the paper that such a plan would cost the leading breast cancer charity �15,000 every year.

"It is extremely unfair to levy VAT on individual donations," he added.

"Supporters would feel very disappointed their donations were benefiting the tax man and not Breakthrough."