Some flights and ferries were cancelled on Saturday amid a “perfect storm” of the Christmas getaway and bad weather.
The disruption has been caused by strong gusts after the Met Office announced yellow warnings for parts of the country.
Heathrow airport said a “small number” of flights had been cancelled owing to strong winds and airspace restrictions.
A spokesperson said: “This will ensure the vast majority of passengers can still safely travel as planned.
“We know how important travel at this time of year is, and have extra colleagues on hand in our terminals to support people on their journeys. We encourage passengers to check in with their airlines for the latest information about their flights.”
P&O Ferries said journeys between Larne in Northern Ireland and Cairnryan in Scotland had been cancelled for at least 24 hours.
A spokesperson said: “Due to adverse weather conditions, all P&O Ferries sailings between Larne and Cairnryan have been cancelled from 8pm tonight (21 December) until at least 8pm tomorrow night (22 December). We ask our customers not to travel to either of the ports if their sailing has been cancelled to avoid causing traffic congestion.”
The AA had predicted 23.7 million drivers would use the roads on Friday, which would make it the busiest day since the group’s records began in 2010.
It projected that on Saturday 22.7 million drivers would be on the road and on Sunday 21.3 million.
A spokesperson for the group said: “With more than 20m car journeys expected on both Saturday and Sunday, if the congestion wasn’t enough of a headache, the inclement weather could create the perfect storm.
“We advise those heading out to allow extra time to travel and increase the distance between themselves and other road users.”
The RAC estimated 7m leisure trips would be made on major roads at the weekend, excluding everyday traffic.
It predicted there would be congestion hotspots in both directions on the M1 to Gatwick via the M25 and the M23; Liverpool to Chester on the M53; Oxford to the south coast via the A34 and the M3; the M25 to the south coast along the M3; and at the Taunton to Almondsbury interchange in Bristol heading down the M5.
Speaking to BBC Radio 4’s Today programme on Saturday morning, the RAC’s senior policy officer, Rod Dennis, urged drivers to be patient: “What we’re saying to people is just be patient. These are journeys that matter to us this time of year. Don’t expect to get there the minute your satnav says it will. Allow yourself a bit of time to make it easy and get there safely.”
Westerly winds were forecast to pick up on Saturday with 50-60mph gusts expected, with a chance of some reaching 80mph.
The Met Office said: “The strongest winds are expected across the far north of Scotland on Saturday afternoon and evening, with the potential for gusts in excess of 80mph in coastal districts including Orkney.
“Dangerous coastal conditions can be expected too, with large waves an additional hazard, especially with respect to causeways. This period of strong winds may lead to some transport disruption, including ferry delays or cancellations.
“Frequent blustery showers will also be a feature on Saturday and may merge into a longer spell of rain for a time in the far north and north-west.”
Showers could turn to several centimetres of snow on the hills in the north-west of Scotland from Saturday evening into Sunday. Some sleet, snow and hail may fall at lower levels and produce icy conditions by Sunday morning. The strong winds will be more widespread on Sunday, meteorologists said.
Sunday’s warning also includes London, the south-east, the south-west, the East Midlands, the West Midlands, Yorkshire, and all of Wales and Northern Ireland.
By noon on Saturday, Caledonian MacBrayne, which operates ferry services on Scotland’s west coast, reported that sailings on 21 of its routes were cancelled for the day, with other services facing disruption.
Northlink Ferries, which runs services between Aberdeen and Orkney and Shetland, also reported disruption. It said “adverse weather conditions” meant a sailing between Kirkwall on Orkney and Lerwick in the Shetland Isles was cancelled, along with sailings between Aberdeen and Lerwick scheduled for Saturday evening.
ScotRail said speed restrictions would be in place for part of the route between Glasgow and Oban, Glasgow and Mallaig, and the Inverness services to Kyle and Wick.