The Met Office says temperatures will keep rising into Friday and the UK's all-time temperature record of 38.5C (101F) - set in Kent in August 2003 - could be beaten.
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.
The Met Office is warning people to stay out of the sun on what's already become the hottest day of the year, reaching more than 33 Celsius in Suffolk, And temperatures are predicted to soar even higher in some places later this week.
A respite from the heat could arrive with thunderstorms in eastern areas.
Tomorrow is tipped to be the hottest day of the year so far with temperatures tipped to topple the monthly record.
The British heatwave has brought the hottest day of the year so far with temperatures soaring to 35C.
The UK's current all-time record temperature for July stands at 36.7C, which was recorded at Heathrow in July 2015.
The all-time record in the United Kingdom is 38.5degC at Faversham on August 10 2003.
A spokesperson for the Met Office said: "With hot air over the United Kingdom some scattered thunderstorms may form over parts of central and eastern England on Thursday afternoon".
"Whilst many places will remain dry and hot, the thunderstorms on Friday could lead to torrential downpours in places with a much as 30mm of rainfall in an hour and 60mm in three hours".
"Large hail and strong, gusty winds will be additional hazards".
It comes after an amber "heat health watch" warning was issued for parts of England with people advised to stay out of the sun for the remainder of the working week.
An amber "heat health watch" warning has been issued for parts of England this week. The Heat Health Watch Service is created to help healthcare professionals manage through periods of extreme temperature.
Temperatures were around 21C in parts of East Anglia and London past midnight.
The level 3 warning, one behind a national emergency, has been issued with temperatures set to be in the high 20s this week.