Temperatures could reach 30C on Thursday, with some of the highest figures expected in the south and east, the Midlands and South West England.
The nights will also be much warmer from Thursday onwards, with many areas seeing temperatures in the high teens.
By Friday, the north and west will be feeling the heat, with Aviemore, Strabane and Aberystwyth among the possible spots to hit 28C or higher.
Northern Ireland, western Scotland and west Wales will see temperatures peak on Friday with highs of 24 to 28C likely.
For east Wales, eastern Scotland and much of England the peak will be on Saturday, where many areas will see highs of 28 to 31C, while some places in south-east England and East Anglia may reach 33C.
Saturday night could see the first “tropical night” of the year. This is where overnight temperatures do not drop below 20C, something that is increasingly likely for parts of south-east England.
The UK Health Security Agency (UKHSA) has issued yellow heat-health alerts for all regions of England except the North East and North West. They came into force at midday on Wednesday and last until 18:00 on Sunday.
While yellow is the lowest level (below amber and red), it warns of possible impacts on health and social services and recommends preparation for vulnerable groups, including the elderly.
