The heatwave is set to continue into Friday - with forecasters saying temperatures could get as high as 37C.
Here are some weather records for the UK and further afield:
- 30.8C: The hottest temperature ever recorded in Northern Ireland, at Shaw's Bridge, Belfast, on July 12 1983 and Kockarevan, Co Fermanagh, on June 30 1976
- 32.9C: The hottest temperature ever recorded in Scotland, on August 9 2003 in Greycrook.
- 33.3C: The hottest temperature ever recorded in Ireland, at Kilkenny Castle on June 26 1887.
- 35.1C: The hottest day of 2018 so far, recorded at Wisley, Surrey
- 35.2C: The highest temperature ever recorded in Wales, on August 2 1990 in Hawarden Bridge, Flintshire
- 36.7C: The hottest July day recorded, on July 1 2015 at Heathrow
- 38.5C: The highest temperature recorded in the UK in Faversham, Kent, on August 10, 2003
- 48C: The hottest temperature recorded in Europe, on July 10 1977 in Athens, Greece
- 58.7C: The hottest temperature ever recorded on Earth, in Furnace Creek, California, on July 10 1913