With turnout set to be a crucial factor in Britain's European Union membership referendum, heavy rain and thunderstorms were causing slow journeys home Thursday evening for thousands of London commuters.

Southeast England was hammered by up to 1.75 inches (4.6 centimeters) of rain overnight — roughly the average for all of June — and another band of thundery rain swept through on Thursday afternoon.

Doug Wilson, a flood risk manager at the Environment Agency, said "torrential downpours across the South East, including London, could lead to further surface water flooding and significant disruption to travel."

A handful of polling stations in London opened late or had to relocate because of flooding, while signal failures caused by flooding shut down sections of the London Underground subway system and several train lines into the capital.

The Rail Delivery Group said train services would be severely disrupted Thursday evening and people should head home early if they can.

That wasn't an option for many, and thousands of people crammed into Waterloo and other London train stations, eyeing signboards full of canceled and delayed trains.

The disruption could hit the turnout for Britain's referendum on whether to stay in the 28-nation EU, since many people go to polling booths after work. London voters are expected to strongly favor remaining in the EU. The polls close at 10 p.m.