Britons are being urged to hydrate and seek shade as the United Kingdom wilts amid a record-breaking spell of hot weather for May that will last until Wednesday.

The UK broke its record on Monday for the hottest day in May, according to the national weather agency, with the mercury rising to 33.5 C near London.

It has been the hottest in the nation's southeast, where BBC forecasters said the temperature could even hit 35 C.

The previous hottest May day saw 32.8 C heat, first registered in 1922 and again in 1944.

The normally temperate UK usually only sees such temperatures at the height of summer.

Monday also happened to be a public holiday and many workers headed for the beach, making roads out of cities unusually busy.

Sunday was also a scorcher, with the 32.3 C recorded at a weather station in West London's Kew Gardens the highest seen in May for 79 years.

It meant that, by Sunday, London had seen temperatures of at least 28 C for three consecutive days, meeting the UK's official definition of a heat wave.

A spokesperson for the national weather and climate service, known as the Met Office, told The Guardian that climate change is playing a part in the unusually hot weather.

"Breaking the 32.8 C May record is around three times more likely now in our current climate than it would have been in natural climate conditions before the Industrial Revolution," the spokesperson said. "What was around a one-in-100-year event is now around a one-in-33-year event."

The heat wave was especially miserable for people in 500 homes in the counties of Sussex and Kent, which were left without tap water because of "increased demand".

The UK Health Security Agency issued an amber health alert for London, Southeast England, East England, and the Midlands that will remain in place until 5 pm on Wednesday, warning there is a likelihood of "a rise in deaths, particularly among those aged 65 and over or with health conditions".

The heat wave is also driving up temperatures in mainland Europe, which is in line with the warnings contained in the latest European State of the Climate report produced by the European Union's Copernicus Climate Change Service and the World Meteorological Organization.