The coach of Britain's Olympic champion Denise Lewis has admitted that he is "very concerned" by the mystery stomach problem that threatens to derail her hopes of taking a first World title.

Charles van Commenee attempted to put on a brave face after announcing that Lewis has only a 50% chance of competing in the heptathlon, which begins on Saturday morning.

But the Dutchman revealed to BBC Sport Online that the problem was far more serious that he had initially hoped.

"There's every reason to be concerned when you're not fit two days before the championships. But showing my concern to everybody won't help," he said.

Van Commenee said that Lewis had first started feeling ill on Wednesday night. She has thrown up a number of times and is feeling weak.

"I'm not always there so I'm not keeping track,"

"It's very difficult to deal with," he admitted. "Denise is down at the moment.

"The heptathlon is about being at your very, very best on Saturday and Sunday. Any disturbance will have a big effect on how you feel and, at the end of the weekend, your score and your ranking.

"It is very frustrating. But the art of sport is to deal with difficulties. Problems happen and it's a matter of dealing with them as you can."

Van Commenee said that Lewis has been suffering from the stomach problem for a number of years, but said there was little she could do but wait for it to pass and "keep her fingers crossed".

"It is something that has happened to her over the past ten years. I'm not saying that it happens all the time, but it is familiar," he said.

"She can wait and see what happens, rest and try to get some meals in, take some medication and get some rest."

Lewis had been due to give a media conference on Thursday.

When it was first announced that she was ill, there were mutterings that it was simply an excuse for her to stay away.

But Van Commenee dismissed talk of such a stunt.

"When you're not feeling very well but you are meant to show up for the media, what choice do you have?" he said.

"You can be very honest, which might encourage your opponents, or you lie about things. Last year she looked on the positive side more, but you can't do that every year."

Lewis has seldom approached a major championship without health concerns of some sort, yet she has still managed to win every major title bar the Worlds.

From this perspective, all is not yet lost.

"It's very rare in the heptathlon for an athlete to produce seven great events," said Van Commenee.

"Sometimes it's good enough if you're just alright. Last year she won an Olympic medal after failing in the high jump.

"Denise hasn't been in her best shape but she's much better now that she was a few weeks ago.

"We managed to get some work in, so she'll be between good and very good in six events.

"She's poor in the high jump, but overall she's got a lot better in the last three weeks."

At last month's British Trials in Birmingham, Lewis was well below par in three of the four events she took part in.

She ran 24.63secs in the 200m against a personal best of 24.10secs, produced just 1.68m in the high jump compared to her PB of 1.87m and threw 45.68m in the javelin (PB 51.13m).

But the heptathlon is not an ordinary event. Athletes will complete just one or two competitions a season, and championships are won by those who are able to raise their game on the crucial two days.

It is also an event which is notoriously hard on the body. Lewis went to the Olympics with injuries, picked up a couple more competing and was forced to take three months off to recover.

"The healing wasn't perfect," admitted Van Commenee, "but it was good enough to start training.

"Three months is a long chunk of time to be out of training so it was difficult. We only had seven months to prepare, but I think we did a good job."