The accusations first surfaced in the United States and Europe, but are now becoming increasingly common in Britain.

Their supporters say race backers are keen to find local winners and that the Kenyans are not "sponsor friendly."

The director of one high-profile race in the US is on record as saying "American sponsors want American winners".

John Bicourt says demands have changed in 10 years

But promoters are keen to shrug off the charges, claiming they simply want to keep a fresh field of runners.

The former British Olympic steeple-chaser, John Bicourt, who manages the careers of around 30 Kenyan athletes, has spoken out about the "discrimination."

Whereas 10 years ago he could have placed his clients in any race, Mr Bicourt claims he now has to fight to get them into meetings.

Although Kenyan men lead the pack - they hold four track world records and have won 13 consecutive team titles in the World Cross-County Championships - he says they fail to draw big crowds.

"Probably the best Kenyan is Daniel Koman, but how many people would have heard of him?" said Mr Bicourt.

John Cain denies discrimination

"You've only got to come down a few notches and there are Kenyans that nobody has heard of. Therefore it's not 'interesting'."

He says the British media are only interested in "British athletes winning British races."

Northumberland's six-day All Terrain Marathon is a case in point, says Mr Bicourt, who entered last year's Kenyan winner, Joseph Kanda.

This year Mr Kanda came back to Britain to defend his title, only to be told he could not take part in the closed competition. Mr Kanda, who won more £4,500 for last year's victory, called the result "unfair".

Joseph Kanda was not allowed back for a second year

But claims of "discrimination" are disputed by Nova International, sponsors of the All Terrain Marathon.

Operations director John Cain says he would love to have a British winner, but that is not why Mr Kanda has been excluded.

He says it is common practice not to include the previous year's winner in the following year's race.

But he omits to say that the event's 1996 champion, British athlete Andrew Pearson, was invited back in 1997 and took part in this year's race.

Athletics commentator Bob Frank says sponsors "like to feel they are getting something out of the athletes in return."

"I find British athletes are more happy to meet with the sponsors and talk to anyone."