The Guoman Tower Hotel said they were "not the source of the illness" following investigations carried out by the IAAF, Public Health England (PHE) and environmental health officers.

Two of the 30 cases have been confirmed as the norovirus, according to health officials.

PHE said they were unable to give an exact number "because norovirus is very easily spread".

The IAAF have revealed that of the 30 athletes and support staff who have reported systems, there are nine current cases of illness.

Members of the Botswana, Germany, Canada and Ireland teams - all of which have had athletes who have contracted the sickness bug - are staying at the hotel.

Canadian team doctor Padraig McCluskey claimed PHE were confident it was not due to food or water at the hotel but the exact source has not yet been identified.

The spread of the virus, usually brought on by a bacterial or viral stomach bug and which causes diarrhoea and vomiting, is also limited to the Tower Hotel, McCluskey added.

"We have followed strict hygiene protocol, ensuring that those affected are not in contact with other guests and all public areas have been thoroughly sanitised," a Tower Hotel statement read.

"We continue to liaise with the medical authorities and the IAAF to ensure the comfort of those guests affected and the health and well-being of all our guests remains a priority."

Those affected include Botswana's Isaac Makwala, forced to withdraw from the 200 metres heats last night.

Makwala has also been withdrawn from tonight's eagerly-anticipated 400m final "due to a medical condition on the instruction of the IAAF Medical Delegate".

This comes despite the 30-year-old, among the favourites for a medal in the 400m, telling the BBC he was "ready to run".

"It would be devastating," he said on the possibility of missing out on the race.

Others affected include Irish 400m hurdler Thomas Barr, who had been due to compete in the semi-finals here last night but pulled out yesterday morning.

Athletics Ireland have confirmed Barr is the only Irish athlete to have been hit with the bug.

A total of nine cases of the sickness bug have been reported in the Canadian contingent here.

McCluskey said around half of those are staff members and not athletes but he did not give a precise figure on the amount of competitors involved.

insidethegames understands some of the athletes affected have been quarantined in the Tower Hotel, the base for the London Marathon since the race started in 1981, in a bid to stop the illness spreading further.

It has been reported that 30 members of the German team who are due to arrive today will be moved to other hotels.

German triple jumper Neele Eckhardt collapsed as a result of contracting the virus but competed in the triple jump final on Monday (August 7).

The rest of the Canadian team travelling to London will also stay at a different hotel, McCluskey said.

This step has also been taken by the Athletics Ireland, who confirmed athletes who have already competed will fly home to "protect their health".

In a statement sent to insidethegames, the Botswana Athletics Association said the IAAF were "doing everything possible to find out more information about the outbreak".

Onkabetse Nkobolo, also a 400m runner, and Makwala are the two cases in the Botswana team.

A spokesperson for the German Athletics Federation told insidethegames they were "analysing the situation together with our own medical team, the IAAF and London 2017".