"Busiest travel day" to be hit by strikes and delays, holidaymakers warned.
Saturday is expected to be the day most people travel abroad.
The "busiest travel day of the weekend" is likely to be hit by train strikes and delays, it has been warned.
Saturday is expected to be the day most people travel abroad, according to travel association Abta, after schools across England and Wales broke up for the summer holidays this week.
But thousands of rail workers are to strike and the Port of Dover has warned it could take up to two and a half hours to pass through border checks during peak hours.
Traffic jams already affected motorists on Friday, and the Met Office has warned of travel disruption as downpours begin on Saturday, with a risk of thunderstorms on Sunday.
The walkout by members of the Rail, Maritime and Transport union on Saturday is due to a long-running dispute over pay, jobs and conditions.
It is expected to disrupt services badly, with some parts of the country having no trains all day.
Services are also being disrupted because of a ban on overtime by train drivers in the Aslef union.
The strike at 14 train companies is expected to see wide variations in services across the country with trains starting later and finishing much earlier than usual.
In some areas only around half of train services are expected to run while others will have no services at all.
Travel trade organisation Abta said more than two million UK holidaymakers will head overseas this weekend.
The Port of Dover said Friday was "an extremely popular travel day" and traffic was "moving according to plan".
By 3.08pm, 20,490 passengers had sailed from the Kent port to France.
Meanwhile, hundreds of thousands of passengers are travelling through Heathrow and Gatwick airports over the coming days.
Abta said about 200,000 people will depart from Manchester airport and 71,000 from Bristol.
And in an effort to help families travel more easily, children aged 10 and over will be able to enter the UK using eGates across 15 air and rail ports from Monday, the Home Office said.
For those holidaying in the UK, the first two weeks of the holidays will be a washout, but there are signals that indicate during the second week of August the weather could start to become more settled, the Met Office said.