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PHOTOS: Travel grinds to a halt in London after nearly 1 month's worth of rain falls on EU referendum polling day

Heavy rain triggered flooding and caused travel chaos in London in the early morning hours of the European Union referendum polling day.

A depression that tracked from northern France to the North Sea spread heavy rain and thunderstorms across South East England on early Thursday morning.

The rain totaled 44.4 mm (1.75 inches) at St. James Park, according to the Met Office. London averages 45.7 mm (1.80 inches) for the entire month of June.

Other rainfall totals include 40.6 mm (1.60 inches) at Hampton Water Works and 34.6 mm (1.36 inches) at Kew Gardens.

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The London Fire Brigade's 999 control officers received more than 550 weather-related emergency calls and firefighters attended over 400 incidents due to the torrential rain and thunderstorms, according to an official press release.

"The high volume of weather-related emergency calls received between 0130 and 0630 this morning is nearly twice the number of all 999 calls that our control officers deal with in a normal day," a London Fire Brigade spokesperson said.

Incidents attended by the Brigade's crews included "flooded commercial and domestic properties, flooded roads, and vehicles that have been trapped by flood water."

Some of the flooding occurred along the River Rom at Romford, where the Environment Agency reports the river came out of its banks in many locations.

"Firefighters also attended a number of calls to reports of property being struck by lightning," the press release said, adding that there have been no reports of serious injuries.

The flooding disrupted rail and tube service. Transport for London reports that service has been suspended on parts of the District Line and London Overground.

After lengthy delays, Southern Railway also suspended all West London services for the remainder of the day.

The travel chaos created issues for commuters and those heading to the polls on European Union referendum day.

BBC News reports a polling station in Chessington sustained flooding but remains open.