The first big getaway of the summer has begun under black skies and torrential downpours, with flash floods making many roads and railways too treacherous for travel.

Passengers have been advised not to travel by rail, as severe weather conditions caused disruption to train services throughout Wales, the South West and southern England.

As families prepare to decamp for the beginning of the school holidays, motorists have been told to avoid all but essential travel, with lorries and vans becoming submerged in town centres, and passengers trapped in waterlogged vehicles.

Weather forecasters predict that the band of rain, which has made this month the wettest July for years, will move northwards up the M4 corridor, saturating the Midlands before moving on to soak Scotland over the weekend. The average rainfall for the whole of July is about 35mm, but some places had as much as 85mm in the space of a few hours yesterday.

Rail services across the country have been disrupted or cancelled after two months of rain fell in a single day in some areas, with the South and South West worst affected. Virgin Trains, First Great Western, Arriva Trains Wales, Chiltern Railways, First Capital Connect, South West Trains, Southeastern and Southern are among the companies affected. Most cancellations are expected on the two arterial rail routes from London to Wales and the West Country.

Trains leaving from Paddington will go no further than Didcot Parkway, or Reading, which will have a knock-on effect on western routes. Service from Birmingham to Coventry, Derby and the South West will also be badly affected by the flooded tracks.

Lydia Aydon, of Network Rail, said: “Most of the problems have been caused by flash flooding. What can happen is that you can have problems with signalling, landslips and lightning strikes, so some services have been cancelled. As soon as the water ebbs away we can start reinstating services but moving slowly at about 5mph.”

Two of four lines in South Croydon have been disrupted by landslips, while passengers have been advised to seek alternative travel because of disruption on the London to Brighton and Doncaster to Leeds routes.

A Virgin spokesman said: “We are putting on replacement buses but our firm advice to passengers is that if they don’t have to travel to and from the South West and to and from Birmingham they should avoid doing so.”

Sewers were unable to cope with the rain and backed up, flooding the homes of some of London’s wealthiest residents. Basements were submerged in many properties in Holland Park, West London, home to millionaires including Sir Richard Branson. A spokeswoman for Thames Water said: “We’ve had about 500 incidents of internal sewer flooding. Because we have so many there can be a few hours delay for engineers to attend.”

The heaviest rainfall yesterday was recorded in Weir Wood, East Sussex, where complaints of drought are usually commonplace. More than 42mm fell in the hour from 10am to 11am. The second highest was 32.2mm in Haywards Heath, West Sussex, an hour later. The wettest overall was RAF Brize Norton, Oxfordshire, with 121.2mm of rainfall in the 17 hours to 5pm. According to meteorologists, the average rainfall yesterday was enough to change the July average rainfall figures for the past 30 years. In contrast, it was in this week last year that Britain enjoyed the hottest day of the month.

The Environment Agency has issued one severe flood warning in Warwickshire, and 19 flood warnings across the Thames Valley, Wiltshire, Yorkshire, the Midlands and Northamptonshire, which will stay in force over the weekend.

Joe Giacomelli, of the Environment Agency, said: “It is unusual to have so many flood warnings in force at this time of the year. If you look back at this time last year, we were battling the drought situation.”

He said the flood defence system had worked well to protect against swollen rivers. Instead, it has been drainage systems that have struggled to cope with sudden deluge. “That’s why you’ve got parks looking like swimming pools. We will be discussing this with the local authorities in due course.” However, he added that there was a silver lining: “The good news is that, contrary to the original weather warnings, the rainband is actually moving across the country fairly quickly. The last thing you want is a situation where you have a slow-moving intense band of rain that just hangs there.”

The latest downpours have already held up the England cricket team’s Test match against India at Lord’s. Dozens of pre-season friendly football matches and several race meetings this weekend could be at risk.

The Truck Festival, which was due to take place on Saturday and Sunday in Oxfordshire, has been cancelled. But the Open Championship at Carnoustie in northeast Scotland looks likely to miss the worst of the weather.

Unusually, the best bet for some sunshine this weekend will be somewhere not known for its clement climate: the Outer Hebrides.