The UK has enjoyed - or endured - the hottest weather of the year so far, with temperatures of 30C in London.
The country is officially experiencing a heatwave, with more 30C days expected later in the week.
Fears that railway tracks could buckle in the heat prompted Network Rail to introduce speed restrictions.
And the Met Office has issued a yellow health warning for heatwave conditions between Tuesday morning and Thursday evening.
The warning, in the west and north-east of England, means there is an 80% chance of temperatures which could be a risk to health, 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."
On Monday the highest temperature of 30.1C was recorded at Hampton waterworks in west London.
The year's previous record temperature had been 29.1C in April at St James's Park in central London.
In Scotland, the highest temperature of 26.7C was recorded in Balmoral, while the same temperature was also reached in Castlederg, Northern Ireland.
Temperatures reached 28.7C in Usk, in south-east Wales.
BBC Weather presenter Matt Taylor said: "We're likely to get 30C on a couple of days this week, and it could still get hotter.
"If Scotland and Northern Ireland get above 29C, which they are likely to do by the end of Wednesday, it will be the hottest June day in both countries since 1995."
Network Rail said track temperatures could reach as much as 20C above the air temperature, meaning the steel rails can expand markedly. 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."
Heatwave health warnings explained
Green - minimum state of vigilance during the summer.
Yellow - Alert and readiness - Social and healthcare services are on standby to reduce harm.
Amber - Heatwave action - Care services target high-risk groups, like the elderly, with specific help. General advice is given to keep homes cool, drink plenty of fluids, and so on.
Red - National emergency - Reached when a heatwave is so severe that illness and death may occur among the fit and healthy, not just among high-risk groups.