It is feared viruses will spread quickly in the huge crowds at the Hajj
British Muslims will need a seasonal flu vaccination before travelling to Saudi Arabia for this year's Hajj pilgrimage, because of swine flu fears.
The Saudi government says all pilgrims need a certificate of vaccination before they can apply for a visa.
The World Health Organisation and the Saudis also say that those in high-risk groups, including pregnant women and the elderly should not travel at all.
Two million Muslims converge on Mecca each year for the Hajj pilgrimage.
Swine flu fears
Pilgrims will also need to be vaccinated against H1N1 swine flu, if a vaccine becomes universally available before the Hajj in November.
The new measures have been introduced because of the risks of viruses spreading in such a large concentration of people.
The Hajj is the journey that every adult Muslim must undertake at least once in their lives if they can afford it and are physically able.
"It is inevitable that at an event where two million people are expected to meet, viruses will spread quickly - and this year there is greater concern about flu, particularly for vulnerable people", said Communities Minister Shahid Malik.
He said any British Muslim planning to travel in November should check the Foreign Office travel advice, or call 0845 850 2829.
Pilgrims are also advised to have comprehensive travel insurance, and to carry vaccination cards with them.
The Foreign Office is sending its own delegation to the Hajj, which will include doctors and consular staff, to help British pilgrims.