Three quarters of the people questioned for a survey about the 2012 Olympic Games said they support London's bid to host the event.
But 48% of the 1,600 people surveyed feared Britain's transport network would not be able to cope.
And 46% were worried that hosting the event could cost too much, according to the study by exhibition company ExCel, which could host part of the event at its 100-acre venue in Docklands.
London officially entered the race earlier this month with its bid being headed by Barbara Cassani.
Transport for London (TfL) insists that because of upgrades and improvements the transport system in the city will ready by 2012.
A spokesman said tube companies, under the controversial Public Private Partnership (PPP), had vowed to increase the number of trains and TfL has predicted there will only be a 1% increase on demand for transport in London during the games.
Some survey results
80% supported London's bid
60% thought tourism would increase
54% believed Britain's infrastructure and sporting facilities would improve
58% felt that hosting the 2012 Olympics would make Britain 'great' again
40% said a combination of the government, businesses, the Greater London Authority, London residents and the general public should pay for the games
This is because they would be held in August when demand traditionally drops, but the spokesman did concede that the increase would be concentrated in east London where the event would be held.
"What we have at the moment, covering the whole Olympic Village area, is four tube lines, Central, Jubilee, District and Metropolitan," the spokesman said.
"There are also two national rail lines and the Docklands Light Railway (DLR).
"We are in discussions about some upgrades on the DLR. We hope to have increased capacity certainly before 2012."
He also said there were plans for more trains on both the Central and Jubilee lines by 2011.
TfL has supported the Mayor of London Ken Livingstone's claim that Crossrail, which will link east and west London, could be ready by 2012, despite Transport Secretary Alistair Darling's recent warning that it would not.