Temperatures are expected to be widely in the high 20s to low 30s in England and Wales on Saturday, while the mercury could rise to 33C or 34C in south Wales.
Highs of 29.5C are expected in Northern Ireland, which would also make it the warmest day of the year there.
Eastern parts of England will see slightly cooler temperatures thanks to an easterly breeze, but most areas will still meet heatwave thresholds.
Even places such as Scotland’s Western Isles could see unusually high temperatures due to the widespread nature of the heatwave.
The 24C forecast in Stornoway may not seem high compared to the heat elsewhere – but this would be only the fourth time in 10 years the town had been so hot.
The amber heat health alerts cover southern England, the Midlands, and East Anglia and will remain in place until 09:00 BST on Monday, the UK Health Security Agency has said.
This means the weather conditions could affect the whole population and the NHS, with travel disruption likely.
Less severe yellow warnings remain in place for northern England on Saturday – meaning the warm weather could affect those who are particularly vulnerable, such as the elderly and those with existing health conditions.
