Millions of people embarking on a bank holiday getaway are facing congested roads and disrupted rail journeys.
The AA predicted that the dash to popular destinations will "start in earnest" from lunchtime on Friday as people finish work early to make the most of the break.
A poll of more than 16,000 motorists found that 43% are likely to be on the road between Friday and Monday, either travelling on holiday or on a day trip.
This equates to around 15 million drivers on leisure journeys this weekend.
Traffic information supplier Inrix reported that the worst congestion blackspots during last year's August bank holiday weekend were the M25 between Junctions 9 and 21 - which runs past Heathrow Airport - and the M5 southbound between Junctions 14 and J22 in the South West.
Motorists on both sections of road endured delays of up to 90 minutes.
Highways England is removing roadworks from 445 miles of motorways and major A roads ahead of the bank holiday period to ease journeys.
Rail firms have issued stark warnings as major engineering will affect several lines.
Virgin Trains issued an alert stating that its services would be "very busy" over the long weekend.
High demand is expected for east coast trains between London King's Cross and Scotland due to passengers seeking alternative routes to and from London because of the closure of London Euston on Saturday and Sunday, as well as Edinburgh's festivals coming to an end.
The operator said that "if customers choose to travel" they should book tickets in advance, make a seat reservation and expect longer journeys.
A further reduction in already reduced London Waterloo services began on Thursday as a major project at the station reaches its conclusion on Monday.
Fewer than half of normal weekday services will operate on Friday as platforms 1-14 are closed.
National Express, the UK's largest coach operator, is putting on an extra 4,000 seats over the weekend to destinations affected by the Euston and Waterloo work.
There is also disruption on routes serving London Bridge, London Liverpool Street, Cardiff and Bristol.
The proportion of Britons planning an overnight UK trip this weekend has risen by more than a third compared with last year's August bank holiday, according to tourism body VisitEngland.
Some 6.9 million adults are planning a domestic break over the last long weekend before Christmas, up from 5.1 million in 2016,
Met Office forecasters predict that much of England and Wales will see mainly dry conditions with sunny spells, and temperatures reaching approximately 24C (75F) in the South.
But parts of northern England, Northern Ireland and Scotland - which does not have a bank holiday this weekend - will see rain or showers at times, with the mercury reaching around 17-21C (63-70F).
Travel trade organisation Abta estimated 2.1 million British holidaymakers will travel abroad over the weekend with Spain, Turkey and Greece among the most popular destinations.
Heathrow expects 243,000 departing passengers on Friday alone, while an estimated 304,000 will fly from Gatwick over the entire weekend.