The UK has experienced its hottest day of the year so far, as temperatures soared to 32.1C on Sunday.A heatwave has officially been declared in eight locations in South East England, which have now met the threshold of 27C for three consecutive days of hot weather.The Met Office said that Santon Downham was the first location to meet the criteria at around 11:30 on Sunday,It was closely followed by Heathrow at around 11:50, whose criteria is three consecutive days at 28C. Temperatures in that area have since climbed further to 30C.Monday’s temperatures are expected to soar even higher to 34C in Greater London and across South East England.People cool off in the water of the River Great Ouse as temperatures climb over the bank holiday weekend during a heatwave (Reuters)Speaking to the Press Association, a Met Office spokesperson said: “We rarely see temperatures above 35C, even in the summer months, so to see temperatures getting close to 35 in May is, as I say, pretty historic.”Saturday was the first time this year that the UK had passed 30C - the last time that happened in May was in 2012.Elsewhere in England, temperatures climbed to between 25 and 30C on Sunday. However, Scotland is experiencing cooler temperatures, reaching the mid-teens.It comes after a series of amber heat health alerts came into force in London, the South East, the East and the Midlands on Friday, while yellow alerts are in place across the rest of England.The warnings mean there is likely to be a “significant” impact on health and social services, and a rise in deaths, according to the UKHSA.With a heatwave developing over the weekend, parts of the country are seeing higher temperatures than those in Mediterranean countries, including Greece.The Met Office is also anticipating the hottest May day on record, with temperatures set to reach 33C in southern England and the Midlands in the coming days - exceeding the current May temperature record of 32.8C.Met Office deputy chief forecaster, Steve Kocher, said: “It is likely that the May and Spring UK temperature records will be broken over the Bank Holiday weekend, with forecast temperatures surpassing the existing record of 32.8C.“As well as it being hot, there will be lots of dry and sunny weather for much of the UK. There will however be more cloud and some showers in Northern Ireland and western Scotland through the weekend. With relatively low sea surface temperatures there is also likely to be some low cloud and sea fog clinging closely to western coasts.”The all-time May temperature record currently stands at 32.8C (Camden Square 22 May 1922; and then Horsham, Tunbridge Wells and Regent’s Park on 29 May 1944).