The hottest July day on record is 36.7C (98F), which was reached at Heathrow on July 1 2015.
But there are also thunderstorm warnings in place for parts of England, bringing the risk of flash flooding.
"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.
The Met Office warning applies for the whole of the east coast of England from 2pm to 11.45pm on Friday, July 27. "Parts of southern England have seen only six per cent".
After weeks without rain there is a chance of flooding on Friday, with as much as 30mm expected to fall in just an hour.
The high temperatures have been blamed for delays to cross-Channel rail services of up to five hours.
"For the United Kingdom as a whole, we've only seen about 20% of the rainfall we'd normally expect throughout the whole summer".
An update will be issued when the alert level changes in any region.
Image Temperatures on parts of the Tube in London nudged 36C
London Fire Commissioner Dany Cotton, who has written to all London councils suggesting the move, said: "I have attended a number of these large grass fires and it never ceases to shock me how many abandoned smouldering barbecues fire crews spot".
Official advice states: "Heatwaves can be risky, especially for the very young or very old or those with chronic disease".
Nurses have warned that some hospital wards have reached more than 30C, leading to patients and relatives passing out or vomiting.
Mr Burkill also said that temperatures above 34C "can't be ruled out".
Kim Sun-Lee, national officer at the RCN, said: "It is vital employers adapt working practices to the heat - both patients and nurses must have easy access to water, and all healthcare staff should be able to take regular breaks, preferably somewhere cool".
The NHS said the hot weather could increase risks to vulnerable patients, such as older people, babies and young children, and people with serious chronic conditions.
Saffron Cordery, deputy chief executive, said: "We have heard concerns about large numbers of people from care homes requiring treatment".
Stay out of the sun.
The Countdown To 21st Century's Longest Total Lunar Eclipse Has Begun!
A super blue blood moon rises behind the 2,500-year-old Parthenon temple on the Acropolis of Athens, Greece, on January 31, 2018. Those of you in the United Kingdom should look for this eclipse starting in at around 11PM (that's 23:00 hours) local time.