Paris Olympics threatened with further chaos as lightning storm warning issued for French capital

It means that certain events, including Andy Murray and Dan Evans' second-round doubles match, as well as the Women's Rugby Sevens medal matches, could be impacted.

Other tennis matches at the Roland Garros stadium, which typically hosts the French Open, along with football, hockey and beach volleyball fixtures, could also be rained off.

Met Office deputy chief meteorologist David Oliver said: "There’s a chance of some thundery showers across some southern areas of England on Wednesday, then on Thursday there is a signal for some potentially very heavy thunderstorms to develop.

"There are still details to confirm during this period, but in any event, there is a chance of some impacts on each day, especially Thursday.

"The heaviest showers on Thursday could result in 20-30mm of rain within an hour, with daily totals possibly reaching as high as 90mm if multiple showers impact the same location. Lightning and hail present additional hazards, with disruption likely for some. This is a developing element of the forecast, so it’s important to stay up-to-date with the latest outlook in the coming days."

Thursday’s thundery breakdown heralds a shift to a more unsettled weather regime for the UK later in the week and into the weekend, with a westerly weather regime likely to bring outbreaks of rain at times, albeit with some drier interludes.

It comes as the men's triathlon will not go ahead as planned later today as pollution levels in the Seine remain too high, dealing a blow to organisers and leaving athletes facing more uncertainty. The race was postponed to Wednesday at 10:45 am, immediately after the women's event, which is scheduled for 8 am that day.

Organisers previously said they were confident water quality would improve in time for the race after heavy rains last Friday and Saturday dirtied the river. A spokesperson said: "Despite the improvement of water quality levels over the last hours, the readings at some points of the swim course are still above the acceptable limits."

Paris authorities have invested heavily to make the Seine swimmable as a key legacy of the Games, and spent €1.4billion (£1.18billion) on wastewater infrastructure to contain sewage and minimise spillage into the waterway.

Paris Mayor Anne Hidalgo took a dip in the river herself earlier this month, in a bid to convince doubters. City authorities have announced plans for three Seine swimming sites to open to the public by June next year.

Should levels of bacteria remain too high by Wednesday morning both the men's and women's races are likely to be postponed to Friday, the contingency day reserved for the events.

If by Friday the water quality is still not good enough the swim leg will be scrapped and athletes will compete in a duathlon instead. For the mixed triathlon relay event on Aug. 5, the contingency day is Aug. 6.