Holidaymakers in Britain have been warned they could face delays when travelling this weekend as the weight of almost 18m car journeys and widespread rail engineering works are expected to cause disruption over the August bank holiday, celebrated in England and Wales.

The RAC said drivers should set off as early as possible to avoid heavy traffic, particularly on major routes to airports and coastal areas. The south-east and south-west of England are expected to see the heaviest congestion.

Heavy traffic is expected on the M5 between Bristol and Devon, according to the transport analytics company Inrix. On Friday and Saturday there are likely to be holdups of more than 40 minutes on the stretch between junction 15 north of Bristol and junction 23 for Bridgewater.

On Friday, drivers making Channel crossings via Dover or Folkestone are expected to face delays exceeding 30 minutes on the M20 in Kent.

The best times to travel will be after 7pm on Friday, before 9am on Saturday and before 11am on Monday, with traffic expected to be lighter on Sunday.

Approximately 17.6m car journeys are expected to be made during the long weekend. The figures are based on a survey of 2,080 UK adults, extrapolated to the 34m cars licensed in the UK.

Nick Mullender, the repairs team leader at the RAC, said: “More traffic on the roads will inevitably lead to more vehicle breakdowns, especially if the sun makes an appearance and people decide on the day to visit popular destinations. Anyone planning routes through these areas should set off as early as possible or be prepared to spend longer in traffic.”

Rail passengers face their own challenges. Some major routes will be closed for engineering works as Network Rail carries out 261 projects over the long weekend.

Long-distance services will be suspended between London King’s Cross and Peterborough on Sunday, affecting Anglo-Scottish routes along the east coast mainline by LNER and Lumo.

Avanti West Coast will run a reduced service to and from London Euston, with no trains between Birmingham New Street and Birmingham International from Saturday to Monday. Affected services by Avanti West Coast and CrossCountry will be diverted, adding extra time to journeys. London Northwestern services will run to and from Birmingham International only.

Helen Hamlin, the chief network operator at Network Rail, said: “While the vast majority of the railway will be running, works on some parts of the network are unfortunately unavoidable as we complete major work to support a better, more reliable railway that we wouldn’t be able to complete in a normal weekend. So we’re asking passengers to check their journeys in advance to ensure their route isn’t affected.”