Traffic levels spiked as millions of people embarked on a bank holiday getaway.
There was also stationary traffic on the A303 near Stonehenge Wiltshire which is a popular route for people travelling towards the South West.
AA president Edmund King described the M25 as “full”, adding that other major roads were “very busy”.
Location technology firm TomTom told the PA news agency that congestion levels in London at noon were at 53%, up from 46% at the same time last week.
Other cities which saw increased jams include Brighton (from 54% to 60%), Manchester (from 36% to 42%) and Leeds (from 36% to 41%).
Congestion levels demonstrate the average length of journey times compared with free-flow conditions.
Transport analytics firm Inrix had urged motorists to avoid setting off for getaway journeys on Friday afternoon due to heavy traffic.
It warned there could be a 25% increase in delays on Friday compared with a typical August day.
Inrix analyst Bob Pishue said: “Drivers hitting the road for one last getaway before the end of summer should expect long delays on key corridors.
“Knowing when and where congestion will build can help drivers avoid the stress of sitting in traffic.”
The RAC estimated that 16.7 million leisure trips on UK roads were planned for between Friday and Monday.
It added that hopes of good late summer weather could see even more people jumping in their car for a day trip or last-minute break.
RAC Breakdown spokesman Rod Dennis said “bumper-to-bumper bank holiday traffic” was expected, especially on the most popular holiday routes.
He urged motorists to carry out essential checks on their car, such as ensuring tyres are properly inflated and have plenty of tread, and that oil, coolant and screenwash are at the correct levels.
National Highways said about 98% of England’s motorways and major A roads were free of roadworks over the weekend.
