* Thursday confirmed as Britain's hottest day of 2018 so far
* Friday could see July and all-time record
* Extreme heat blamed for Eurotunnel Le Shuttle delays (Adds confirmation of hottest day of the year, Eurotunnel)
LONDON, July 26 (Reuters) - Britons sweltered in a prolonged heatwave on Thursday, with temperatures hitting a year high and set to test national records on Friday, the Meteorological Office said.
The temperature reached 34.9 degrees Celsius at Heathrow Airport, officially making it the hottest day of the year so far, the Met Office said.
It forecast a 20-30 percent chance of Britain seeing an all-time highest temperature this week and a 70 percent chance of a record high for July.
The all-time British record is 38.5C set in August 2003 near Faversham in Kent, southeast England, while the highest July temperature was 36.7C in 2015 at Heathrow airport.
"There is greater potential for both the July and the other (all-time) record to go tomorrow," said a Met Office spokesman noting that the highest temperatures are expected in the southeast - in Kent, Essex, Suffolk and Cambridgeshire.
"It's going to depend on the development of thunderstorms ... If the thunderstorms develop early that's going to prevent some of the heat from the sun getting to the ground which will curb the rise of temperatures," he said.
Britain's heatwave has turned green grass brown, triggered an impending hose pipe ban in the northwest of England, and prompted a warning from the government for people to keep out of the sun. Farmers have also warned of possible food shortages later this year.
On Thursday the EuroTunnel Le Shuttle operation warned passengers heading for the Continent at its Folkestone terminal to expect four-hour delays to booked times, blaming restricted shuttle capacity. It said the extreme heat was affecting the air conditioning on board its trains.
The Met Office said that while Britain is expected to see some rain on Saturday and Sunday, the very hot spell will continue next week with temperatures rising to over 30C.
Separately on Thursday a committee of lawmakers said premature deaths from heatwaves in Britain could more than treble to around 7,000 a year by mid-century if the government does not take action. (Reporting by James Davey; editing by Stephen Addison)
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