The opening of the Olympic Games and the summer holiday getaway could create "the perfect traffic storm", a roads' information company has said.
Londoners will face increased traffic congestion of 33% during the Games, predicted the Inrix company.
Traffic on main routes will slow to just 12mph in late July and early August.
It said the first three days of the Olympics posed the greatest risk of traffic troubles.
Inrix added that the Olympic opening ceremony on Friday July 27 clashes with one of the busiest holiday getaway weekends of the summer when traffic levels are already around 30% above average.
This event is followed in quick succession by the men's road cycling event, which will "close thousands of roads throughout south-west London and Surrey".
Inrix said a trial event for the race in August 2011 "resulted in traffic chaos, when journeys of just a few miles took several hours and some drivers were forced to abandon their cars".
Inrix traffic analyst Greg Hallsworth said: "Traditionally, the weekend of July 27-29 is one of the busiest holiday getaway weekends of the year.
"Combine this with the Olympic opening ceremony and the road cycling race and we could have the perfect traffic storm."
He went on: "Nearly 100,000 ticket-holders are expected at the opening ceremony, with tens of thousands of visitors anticipated at the live sites at Hyde Park and Victoria Park. Couple this with the thousands of roads closed for the cycling event and the result is huge stress on the UK's road networks."
