The UK experienced the hottest weather of the year so far, with temperatures reaching up to 29.4C in London.
Forecasters say higher temperatures are expected in the next few days, with 30C temperatures a possibility in the south of England and Wales.
Fears that railway tracks could buckle in the heat prompted Network Rail to introduce speed restrictions.
And the Met Office has issued a yellow warning for heatwave conditions between Tuesday morning and Thursday evening.
The warning, which applies to the west and north-east of England, means there is an 80% chance of heatwave temperatures, and is intended to help social and healthcare teams prepare.
Met Office forecaster Mark Foster said: “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 burned in the short term.”
On Monday the highest temperature was recorded in St James’s Park in central London. The same location had experienced the year’s previous record temperature of 29.1C in April.
Network Rail said that steel expands markedly as track temperatures can reach as much as 20C above the air temperature. The reduced speeds are intended to lower the force exerted on the track, reducing the risk of buckling.
One route affected by the speed limits is the line between London Waterloo and New Malden in south-west London, where trains will be slowed after 11am each day until Friday.
Andy Thomas, managing director of England and Wales at Network Rail, said: “Our engineers and specialist extreme weather teams are monitoring track-side temperatures at vulnerable locations.”