British Muslims may be required to provide flu vaccination certificates if they want to participate in the annual Hajj pilgrimage to Mecca at the end of November, Saudi Arabia has warned.

The pilgrimage, the largest and densest international gathering in the world, presents unique hazards for the spread of the swine flu virus. The H1N1 vaccine is the best method of limiting its transmission, said Ziad Memish, assistant deputy minister for preventative medicine in Saudi Arabia.

More than 2.5 million Muslim pilgrims from over 160 countries will descend on Saudi Arabia between 25 and 29 November, congregating in huge crowds where the flu virus can spread more easily, Dr Memish said.

The Saudi government has asked Britain whether the vaccine will be available to Muslims two weeks prior to the Hajj. If so, British pilgrims may require proof of vaccination from British authorities in order to receive a visa.

The Health Protection Agency (HPA) said yesterday that there were an estimated 78,000 new cases of swine flu in England in the past week. This is up from 53,000 recorded the week before, but falls short of the 100,000 cases seen at the peak in July.

There are currently 751 people in hospital with the virus. The number of swine flu deaths in England now stands at 97, with seven in Wales, 25 in Scotland and eight in Northern Ireland.

Although the vaccine is being delivered to GP surgeries, it will only be available to high-risk groups by mid-December. People with heart disease, cancer, asthma and diabetes are first in line for the jabs, alongside pregnant women and frontline NHS staff. Saudi Arabia has asked whether the vaccine will also be available to Hajj pilgrims.

"We are hoping that the countries sending the pilgrims will comply with giving the vaccine. And we think that would probably be the best measure to prevent the spread of the disease outside the country," Dr Memish said.

In a policy report published in the journal Science, Dr Memish and his colleagues warned that the Hajj represents a "public health security challenge of extraordinary dimensions" because so many people from around the world will be gathering in large crowds.

"Hajj-related exportation of [the] H1N1 virus by returning pilgrims could potentially initiate waves of outbreaks worldwide and burden healthcare systems. No region can be considered free from risk. For example, pilgrims originating from North America (more than 15,000) and Europe (more than 45,000) pass through major airline hubs of the world on their journey, which increases the risk of international spread of the virus," the authors said.

Hajj in numbers

2-3m The number of Muslims who make the pilgrimage to Mecca each year.

23,000 The number of Britons who made the journey last year 49 Number of pebbles each pilgrim casts at Jamarat pillars to 'stone' the devil.