Motorists are being cautioned about potential lengthy hold-ups on key roads as an estimated 17.6 million car journeys are anticipated during the UK's August bank holiday weekend.

The RAC has sounded the alarm, advising drivers to depart at the crack of dawn or "be prepared to spend longer in traffic". The South East and South West are bracing for some of the heaviest motorway traffic jams.

Around 3 million trips for holidays or day outings are expected on Friday as drivers seek to capitalise on the final extended weekend before Christmas in England, Wales and Northern Ireland, according to the RAC. The number is set to increase to 3.4 million on Saturday, 2.4 million on Sunday, and 2.7 million on Monday.

The RAC also noted that an extra 6.1 million motorists have plans for a leisure journey at some point from Friday to Monday. These figures are derived from a survey of 2,080 UK adults and then projected to the 34 million cars registered in the UK.

Transport analytics firm Inrix has forecasted that the M5 between Bristol and Devon will experience the most intense getaway congestion, with the section from junction 15 north of Bristol to junction 23 for Bridgwater likely to face delays exceeding 40 minutes on Friday and Saturday.

On Friday, the M20 in Kent is also predicted to see hold-ups surpassing half an hour, a route heavily used by vehicles heading for Channel crossings via Dover or Folkestone.

The alert covers journeys from junction 7 near Maidstone to junction 3 (Addington Interchange), and from junction 1 at Swanley to junction 5 at Aylesford.

RAC mobile servicing and repairs team leader Nick Mullender said: "With this bank holiday being the last opportunity to enjoy a long weekend before Christmas, our study shows a real eagerness to get out and about with nearly 18 million drivers planning getaway trips.

"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.

"We're expecting major roads to airports and coastal destinations to be extremely busy, especially the south-east and south-west regions which could end up bearing the brunt of most holiday hold-ups.

"Anyone planning routes through these areas should set off as early as possible or be prepared to spend longer in traffic."

Rail travellers, meanwhile, face disruption as key routes will be shut for engineering work whilst Network Rail carries out 261 projects across Britain.

Long-distance services between London King's Cross and Peterborough will be suspended on Sunday, affecting Anglo-Scottish journeys by LNER and Lumo on the East Coast Main Line.

Avanti West Coast will run a limited and altered service to and from London Euston.

Services between Birmingham New Street and Birmingham International will be completely suspended from Saturday through to Monday. Affected Avanti West Coast and CrossCountry services will be rerouted, leading to longer travel times, while London Northwestern services will only operate to and from Birmingham International.

Helen Hamlin, Network Rail's chief network operator, commented: "With the August bank holiday weekend approaching, we've carefully planned our engineering work to minimise disruption, allowing people to rely on the railway and make the most of the late summer bank holiday weekend.

"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."