They say the company messed up a move they posted on a special website where the world champion was taking on all comers.

The participants say the mistake cost them at least a highly-respectable draw and may even have denied them victory.

But Microsoft rejects criticism of the way it has managed the four-month-long Kasparov vs the World game, saying the cyberclash has been conducted within the rules.

In the game, which started on 21 June, Kasparov holds the white pieces and makes a move every other day. The World plays with black and its pieces are moved on the basis of a vote by internet users. They are offered a range of options posted on the Microsoft site by several young chess experts, who also receive suggestions from net surfers.

The row centres on move 58. One of the options, proposed by 15-year-old US women's chess champion Irina Krush, did not appear, because she had server problems when posting the move by e-mail. Consequently, internet users could not vote for her suggestion and chose what turned out to be an inferior option.

'Vote stuffing'

A very vocal group of participants is now complaining of "vote stuffing".

"Fiasco farce chess game" and "you have destroyed a work of art" were among the messages posted on an electronic bulletin board on the Microsoft game's website. As a protest, some users have since tried to vote for a suicidal move which would inevitably lead to the loss of the black queen.

However, other messages posted on the bulletin board have been supportive of Microsoft and have urged players not to resign the game just yet.

Irina Krush herself, who has had a large following on the site, has apparently "retired" from the game.

"We have the ability to track the votes and we've done that any time there's been any kind of allegation of vote stuffing," Diane McDade of Microsoft's Gaming Zone said. "We've bent over backward to be impartial."

The game, described by Kasparov and Microsoft as an experiment for chess playing on the internet, has been closely fought. It has lasted longer than many experts expected and the contest has reached the 60th move in an endgame involving only the Queens, Kings and a pawn on each side, one of the most complex endings in chess.