As the annual Hajj pilgrimage begins in Saudi Arabia, the Saudi government says it expects 40% fewer pilgrims this year, mainly because of fears about swine flu.
The admission is the first official recognition of the likely impact of people's concerns about catching flu.
About two million Muslims converge on Mecca each year for the pilgrimage.
Saudi health authorities announced at the weekend that four people attending the Hajj had died from the H1N1 virus, but played down the risk to other pilgrims.
The BBC reports that the four who died were said to be a Sudanese man, a Moroccan woman and an Indian man - all aged over 75 - and a girl aged 17 from Nigeria.
A health ministry statement said the four had not followed "recommended procedures, especially vaccination against swine flu". It said 16 other cases had been detected and four people were in a critical condition.
The hajj has been the scene of several tragic accidents caused by huge crowds. In 2006, 364 people were killed in a stampede at the entrance to the Jamarat bridge in Mina.
In a bid to avoid a repeat of the disasters, authorities have just completed the rebuilding of the 950m-long five-storey bridge.