Temperatures in some parts of the UK exceeded those in Los Angeles and the Bahamas on Monday as the hottest day of the year so far gripped the country.

Temperatures rose to a sweltering 32.4C in parts of greater London in the afternoon, just above the 32.1C highs of the weekend. Most of England and Wales was affected by the hot weather, which was made particularly sweaty by the absence of a breeze, though the north of England and Scotland saw cooler, cloudier conditions.

Emma Sharples, a spokeswoman at the Met Office, said: “Heatwave is a funny term – we don’t really have a definition of it in the UK – but none of us can deny it has been a prolonged period of hot weather, and night-time temperatures have been quite unusually high, which can also be the cause of potential health problems, as well as a lot of restless nights.”

Public Health England has issued an amber heat alert until Wednesday, one tier below level four, which marks a national emergency.

The warm, dry weather contrasts with the heavy rain that dominated the final weeks of June last year, causing treacherously muddy conditions at the Glastonbury festival, where the highest temperature recorded for the month was just 28C. The forecast for Glastonbury this year looks considerably dryer, but still variable, with the Met Office advising people to bring “both suncream and wellies”.

The hottest day of the year could also still be to come, with predictions that the south of England could have temperatures of 34C on Wednesday and Thursday.

“Traditionally, you’d expect to get these sorts of temperatures much later on in the summer, in late July and August, but we’re seeing high temperatures already,” said Sharples. “And we’ve already had a fine spell of weather in May.”

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The unusually warm weather is due to an air mass coming from the tropical parts of the Atlantic, around the Azores islands, while at the same time the very hot conditions across France and Spain last week are being pushed northwards.

As the wind changes direction on Wednesday, there will be no respite from the heat. Sharples warned people to expect a few more sweaty, sleepless nights until Friday, with temperatures remaining high even after the sun has set. Night-time temperatures on Sunday and Monday hovered at around 24C.

There will be some thunderstorms across the north on Wednesday and a few showers, though they will get no further south than the Midlands.

The temperatures look set to drop by next weekend, however, as the cool air comes down from Scotland over Manchester and the north of England on Thursday, and over the London and the south-east by Friday.

While the prolonged heatwave will give a welcome boost to sales of sunscreen, ice cream and fans, it also comes with a health risk for the ill, vulnerable and elderly. The 10-day heatwave in 2003 resulted in about 2,000 heat-related deaths, and 680 people died during the long spell of hot weather in 2006.