Temperatures are predicted to reach 35C midweek, threatening the record for the hottest July day, which was set at Heathrow in 2015 with 36.7C.
Monday was the hottest day so far of 2018 in the United Kingdom with a high of 33.3C recorded at Santon Downham in Suffolk.
Rail journeys are being disrupted by the hot weather as train speeds are reduced to stop tracks from buckling, with trains on Chiltern Railways between London and Oxford are being rescheduled because of high track temperatures.
Mr Burkhill added: "The heat and humidity means there is a lot of energy in the air and a high risk of intense thunderstorms".
The all-time United Kingdom high is 38.5C set in August 2003, while the highest recorded temperature for July is 36.7 C at Heathrow on July 1, 2015. An amber warning is triggered when there are at least two consecutive days of at least 30 degree heat in daytime and 15 degrees at night.
Nobby the polar bear cools off at the Yorkshire Wildlife Park Danny Lawson Press Association
Experts at the Met Office have issues a Level 3 heatwave warning for parts of the United Kingdom - just one step away from declaring a national emergency.
There is a 90% probability of heatwave conditions from 9am today until 9am on Friday.
The Met Office has issued a thunderstorm warning for eastern and northern parts of England from Friday afternoon.
A Level 3 heatwave alert has been issued for Surrey and Hampshire by the Met Office.
The dry spell has been most prolonged in East Anglia and south-east England where the last day of very widespread rainfall was seen on May 29. The TUC suggested allowing staff to work flexible hours to avoid travelling at peak times and letting workers wear lightweight clothes. He urged employers to do all they could to keep temperatures down.
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