The warning is in place until 9am on Friday for parts of England as temperatures are expected to rise further.
Official guidance is to “stay out of the sun” and keep your home as cool as possible, shading windows and shutting them during the day.
An amber heatwave action is triggered when the Met Office confirms threshold temperatures for one or more regions have been reached for one day and the following night – and the forecast for the next day has a greater than 90% confidence level that the day threshold temperature will be met.
The Met Office forecasts “hot and humid” air for the east and southeast of England, long spells of sunshine ranging from “very warm” in West and East Midlands, to “hot and humid” and “very hot” in east and southeastern areas.
There will also be “very warm and muggy overnight conditions”, so advice is to open windows at night.
The heatwave is Europe-wide as extreme heat is affecting farmers in Poland, Sweden and Finland.
Sweden is seeing its hottest summer for a century, and temperatures in Finland have topped their typical highs of 18C (64.4F), reaching 30C in places.