With temperatures as high as 28C predicted for the Bank Holiday weekend, millions of travellers will take to the roads, railways and skies.
The RAC warns that Friday will be the peak on the roads. Key routes include the M4, M5 and A303 towards south-west England, routes from northern England to North Wales, the M6 and A590 around the Lake District and the M55 to Blackpool.
Heavy traffic is forecast for Saturday and Monday, with Sunday much quieter.
Rail passengers face multiple problems due to scheduled engineering works on key inter-city lines.
The Great Western main line from Paddington to the West of England, the Cotswolds and South Wales will be severely disrupted from Saturday until 5am on Tuesday morning.
Due to electrification work, trains to Bristol, Cardiff and Exeter will be diverted via the Chiltern line to Oxford, with greatly extended journey times.
On Sunday, Paddington station will be closed altogether, with trains running to and from London Marylebone. Chiltern Railways services will be reduced because of the additional trains. The Heathrow Express will not run.
On the other side of the capital, Greater Anglia services to and from London Liverpool Street will be affected by work to renew the overhead lines. No trains will run between Shenfield and Southend from Saturday to Monday, and on Sunday the line between London Liverpool Street and Ingatestone will be closed.
Passengers travelling to and from Stansted Airport faced problems on Thursday afternoon and evening because of defective track at Cheshunt.
Members of the RMT union working for Greater Anglia will strike on Saturday in a dispute over the role of guards.
The train operator says all planned services will run, using “contingency conductors”.
On the main line from London via Gatwick Airport to Brighton, buses will replace trains south of the airport to Three Bridges on both Sunday and Monday.
Outside the south-east, the worst-hit route is between Crewe and Liverpool, which is closed between Saturday and Tuesday.
Virgin Trains services are diverted via Stockport or Warrington. “Both diversionary routes mean that journeys will take longer than usual,” says Network Rail.
London Northwestern services from Crewe to Liverpool Lime Street will be replaced by buses to Liverpool South Parkway.
On Thursday, buses replaced trains on the line between Oxenholme and Windermere due to a shortage of drivers. It is not known if problems will continue on Friday and into the weekend.
In Scotland, the line between Perth and Larbert, south of Stirling, is closed on Sunday, with passengers facing long rail diversions or replacement bus services.
Travellers to, from and through France face a second day of air and rail strikes on Friday. Air France is cancelling one in four flights as pilots, cabin crew and ground staff continue their industrial action in support of a pay claim.
Flights from Beijing, Buenos Aires, Lagos, Mexico City and Taipei to Paris have been cancelled, and many more long-haul services are expected to be grounded.
Ten flights between Heathrow and Paris Charles de Gaulle have been cancelled on Friday.
Options for travellers between London and the French capital are restricted because Eurostar has cancelled two round-trips by rail because of the dispute on French railways.
On the French rail network, around half of the high-speed trains (TGVs) are likely to be cancelled on Friday as part of a three-month campaign of two days striking/three days working in protest against changes to working practices.
Over the weekend, a strike by air-traffic controllers in the Marseille Area Control Centre will disrupt flights to, from and over southern France. The centre controls airspace from the Swiss border to the Spanish border, and into the Mediterranean as far south as the southern tip of Sardinia.
Italian air-traffic controllers are also planning a nationwide strike between 10am and 6pm, local time, on Tuesday 8 May.
While intercontinental flights and some services to and from the Italian islands are unaffected, many short-haul departures are likely to be delayed or cancelled.