Commuters faced a morning of travel misery after torrential rain lashed parts of the country overnight.
Weather warnings remain in place for much of Tuesday, with forecasters predicting that some areas of the UK could be inundated with up to 4in (100mm) of rain this week.
The downpours led to flooding across roads and railway lines, with some areas being hit with more than a month’s worth of rain in a day.
Heavy flooding in Kent closed all lines through Orpington railway station for several hours, while tracks at Mitcham Junction in south London were also submerged.
Network Rail said that more than a month’s worth of rain had fallen across the South East region in one day, leading to flooding and problems with its power-supply and signalling equipment.
It tweeted that a “conductor rail which powers trains was completely underwater in Mitcham and it also caused a fire in a trackside electrical hut”.
Network Rail South East added: “Across the region, we suffered over a month’s rainfall in just one day and in some areas, the downpours equated to 2 month’s rain.
“We’ve been constantly monitoring the weather and despite handling the brunt yesterday, the deluge overnight overwhelmed some drainage systems.
“However, despite our efforts we still suffered a number of flood issues.”
Rail operator Southern warned passengers to expect delays and cancellations for the rest of the day across the Southern and Thameslink network and urged people avoid travelling in the south London area if possible.
It added: “Trains across the Southern and Thameslink network will be disrupted due to heavy flooding on the railway.
“Southern and Thameslink advise you to avoid travelling on their services in the south London area if possible, especially those via Sutton, West Croydon or Balham.”
Police shut a section of the M25 overnight when two sinkholes were discovered on the central reservation.
Kent Police’s Roads Policing Unit tweeted: “M25 jct 4-5 closed as a result of a single vehicle RTC which in turn has lead to the discovery of 2 sinkholes in the central Reservation.”
The motorway was closed in both directions but Highways England said the sinkholes had been repaired and the road reopened by around 6am.
But the M25 was closed again in both directions between junctions 3 and 4 after police were called to reports of a distressed man on a footbridge above the motorway near Swanley, Kent, at 5.17am.
Kent Police said the man left the scene but was arrested in connection with the incident following a short pursuit on the M2, and the M25 was reopened at about 9.30am.
North-eastern parts of England and the Midlands are expected to bear the brunt of the downpours on Tuesday, with a yellow warning in place until midnight.
Another warning covers Devon and Somerset, where similar conditions are expected.
The Environment Agency has issued three flood warnings in south-east England, for the River Wandle in Morden, south London, the Emm Brook in Wokingham, Berkshire, and the River Ravensbourne in Bromley, south-east London.
A total of 37 flood alerts – which mean flooding is possible – are in place across the country.
On Wednesday and Thursday, some parts of the UK could be struck by 2.4in to 3.2in (60mm to 80mm) of rain, and possibly even up to 4in (100mm), the Met Office said.
Met Office meteorologist Alex Burkill described the figures as the “worst-case scenarios” but added that people need “to be aware that we’re in for some treacherous weather”.
“If you add it all up, some places are likely to see over 100mm this week, which is around double the average they would get in the whole of June,” he said.
Residents are being told to avoid low-lying footpaths near local watercourses and plan driving routes to avoid low-lying roads near rivers which may be flooded.