The Olympic Delivery Authority (ODA) has identified four sites as high risk of becoming waterlogged.
They include parking sites at the sailing venue Weymouth & Portland, at the mountain biking course at Hadleigh Farm in Essex and some sites for football in Glasgow and Coventry.
A replacement site has been found for Hadleigh Farm and one is being assessed for Weymouth & Portland. Those ticket-holders booked into a dropped site will be contacted with details of the alternative parking. Extra shuttle bus services will be laid on from city centres to take visitors to the Coventry and Glasgow venues.
The London 2012 Olympics aims to be a public transport Games, with park and ride facilities helping people travelling by car to complete their journeys to the venues.
Hugh Sumner, director of transport at the ODA, said: "Even before the worst of the summer rain hit Britain, we were reviewing whether our park-and-ride and park-and-walk sites were robust enough to stay open, come what may. The steps we have taken will ensure that there is sufficient provision for everyone who wants to use their car for part of the journey, and that existing sites do not become waterlogged even if there is more rain."
He added: "The message to spectators remains the same – book your travel tickets now, arrive early and check the site for updates regularly."
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