The number of swine flu cases in London is rising rapidly
A UK swine flu epidemic could severely disrupt transport in London, a business group and union have warned.
If 40% of the UK population contract it, as the government estimates, there will be "considerable" travel problems, the Business Continuity Institute says.
The Rail, Maritime and Transport union said the impact would be "devastating".
Transport for London (TfL) said it had contingency plans and any service reduction would correspond with a reduced number of passengers.
There have now been 17 swine flu-related deaths in the UK, including six-year-old Chloe Buckley, from west London, who died on 9 July.
England's chief medical officer Sir Liam Donaldson said the number of cases in London is approaching epidemic levels.
BCI director Lyndon Bird says transport problems will frustrate passengers and firms
Business Continuity Institute (BCI) director Lyndon Bird said he expected "quite a lot" of rail stations to close and a significant reduction in the number of trains and buses available as swine flu spreads.
He said: "This will maybe handle the volume that are required but it will certainly cause considerable travelling delays to the public and will probably cause frustrations and difficulties for businesses."
Rail, Maritime and Transport (RMT) general secretary Bob Crow said transport workers are at greater risk of contracting swine flu as they tend to work in close contact with the public.
Mr Crow said: "If the predicted development of the swine flu is accurate it will have a devastating impact on transport services.
"London is expected to be hardest hit and it would be a dangerous gamble to try to run services without adequate staffing levels."
A TfL spokesman said: "If an epidemic were to occur then it is likely that transport staff, like the rest of the general population, will be affected, which could reduce the number of frontline staff available to operate services.
"But this would correspond with a reduction in passenger demand due to the spread of flu within the general population."
But Mr Crow said this was "dangerous nonsense" that was "miles out of touch with the safety implications of running services with inadequate staffing levels".
However, the TfL spokesman insisted: "The safety of customers and staff is our top priority and is at the core of our contingency planning."