Bookmakers are offering odds on this May becoming the hottest ever recorded, as a “tropical continental air mass” sweeping over the south will make parts of the UK hotter than Portugal.

The south east of England is forecast to reach temperatures of 23 degrees Celsius on Monday, with London, East Anglia and Kent expected to enjoy a heat of 24C, nearly 10C above the average temperature for this month.

Bookmakers Coral have already started offering odds of 2-1 on this May becoming the hottest on record for the UK, and 20-1 that it becomes the wettest on record. William Hill has set odds at 20-1 that temperatures will push 40C at some point this month.

A band of rain moving across northern England and Wales will disappear this afternoon bringing sunny spells for the rest of the country, with highs of 16 or 17 across Scotland and Northern Ireland.

A spokesperson for the Met Office said the hot weather was from a tropical continental air mass moving in across the south, but that the warm temperatures across the rest of the UK are “generally due to a maritime air mass settling in”.

The unusually hot weather will not last beyond the start of the week however, with the Met Office warning that temperatures will feel five or six degrees cooler by Tuesday, though hovering above average at around 19C in the south and lower still at around 17C or 18C on Wednesday.

The spokesperson warned that by Thursday a “low pressure system coming from the Atlantic will bring a band of rain across England and Wales, leaving the day feeling cooler with wet and windy weather, struggling to reach around 14C or 15C”.

The UK enjoyed its sunniest April on record last month, the Met Office said, but the highest temperature of the year has yet to be beaten, which was recorded on 15 April in Faversham, Kent, where temperatures reached 25.6C.