An amber heat health alert has been issued across England, as temperatures are expected to reach above 30C today.
According to the Met Office's latest forecasts, much of the south of England will see temperatures between 31C (87.8F) and 32C by Thursday afternoon, with temperatures above 20C across almost all of the UK.
It would make for the hottest day of the year so far, beating the 29.4C (85F) recorded last Friday in Suffolk and coming after record-breaking spring heat.
Check the weather forecast where you are
By this weekend, temperatures are forecast to hit highs of 33C (91F). The UK Health Security Agency's (UKHSA) latest heat health warnings for England include all regions, and are in effect from midday on Thursday to 9am on Monday.
The amber alert indicates that "significant impacts are likely" across health and social care services, and warns there could be "a rise in deaths" among people with pre-existing health conditions and those aged over 65.
Read more: What is a heat health alert and when is one issued?
A heatwave is officially declared when temperatures exceed a certain level for three days in a row. Thresholds vary from 25C to 28C (77F to 82F) across the UK.
According to the Met Office, it will get progressively warmer as the week progresses - with the heat peaking on Sunday.
Deputy chief meteorologist Dan Holley said thundery showers may be possible heading into Saturday morning, with "tropical nights" a possibility as parts of the UK approach heatwave territory.
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