Britain's hottest day of the year so far has caused travel delays for commuters and sparked wildfires.
Speed restrictions have been introduced on Britain's railways amid concern that rails will buckle as temperatures soar, causing long waits for those travelling by train.
Forecasters have predicted that the the heatwave will continue throughout the week.
Network Rail's extreme weather action teams are being activated across the country to monitor "vulnerable locations".
One route affected is the line from London Waterloo to New Malden, where services are being subjected to speed restrictions from 11am each day until Friday.
Andy Thomas, managing director of England and Wales at Network Rail, said: "On very sunny days, rails in direct sunshine can be as much as 20C above air temperature, causing the steel to expand markedly, and could - if not carefully monitored and action taken - buckle, causing travel disruption.
"Our engineers and specialist extreme weather teams are monitoring track-side temperatures at vulnerable locations and will, if necessary, introduce temporary speed restrictions during the hottest part of the day to keep trains running, albeit more slowly than normal."
Some sections of rails are painted white so they absorb less heat. Slower trains exert lower forces on the track, reducing the likelihood of buckling.
Firefighters tackled a large moor wildfire earlier today in Saddleworth.
Fire crews attended the blaze, which was located between Dove Stone Reservoir and Carrbrook in Tameside.
The fire initially broke out last night and was brought under control, but the flames took hold again on Monday morning due to the hot and dry conditions.
Following a weekend of wall-to-wall sunshine around the UK, temperatures in London's St James Park peaked at 29.4 C on Monday, in line with sunny holiday hot spots in southern Spain.
Very high levels of pollen and UV mean that hay fever sufferers are in for a difficult time.
The previous highest temperature recorded in the UK was 29.1 Celsius at St James Park, central London, on April 19.
The top temperatures in Scotland and Northern Ireland this year were both recorded on May 29.
Achnagart in Ross and Cromarty, Scotland, boasted a temperature of 27.5C while the heat hit 25.3C in Castlederg in Co Tyrone.
People in Hawarden, Clwyd, enjoyed 26.8C, the top temperature in Wales this year, on May 27.
Temperatures on Monday were expected to reach around 29C around in London and between 24C to 27C across England and Wales while Scotland in the south and south east could see around 25C to 26C and Northern Ireland is looking at 25C.
Higher temperatures are expected over the next days and the south west of England, the south and parts of Wales could enjoy 30C.
Urging everyone to take precautions, Met Office forecaster Mark Foster: "High pressure is going to dominate this week so we can expect very high levels of pollen and UV. If you are in the sun you have a greater chance of getting burnt in the short term."
"Long days, very still conditions and clear skies help June temperatures to get very intense. The sun in June is relatively the highest it gets in the sky and heat can build up over successive days."
The TUC has called on bosses to make sure staff working outdoors are protected from the sun and the heat.
TUC general secretary Frances O'Grady said: "We all love to see the sunshine, but working outdoors in sweltering conditions can be unbearable and dangerous.
"Bosses must ensure their staff are protected with regular breaks, lots of fluids, plenty of sunscreen and the right protective clothing."