The opening of the Olympic Games and the summer holiday getaway could create "the perfect traffic storm", a roads' information company said today.
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."
Inrix said journeys with an average travel time of one hour were predicted to take at least 12 additional minutes throughout Greater London for the duration of he Games.
Journeys through routes identified as Games hotspots such as Vauxhall Bridge and the Blackwall Tunnel would take at least 15 minutes longer.
The company also reckoned that in early August the start time for daily congested periods would move forward by as much as 90 minutes, with the morning rush-hour beginning as early as 5.30am and the evening peak as early as 3.30pm.
However, the evening peak could ease by 6pm rather than by 7.30pm as people headed to meeting places to watch events on TV.
Inrix also said that combined with rush hour traffic, Londoners should also anticipate increased traffic around lunchtime as venues clear and refill between Olympic sessions - adding an anticipated nine minutes to standard lunchtime travel times.
Mr Hallsworth added: "UK holiday routes are busier in July and August but in holiday periods, commuting routes are naturally quieter.
"This will assist the London travel network and planners are also hoping for some additional capacity as commuters change their travel routines."