A heatwave is expected to be officially declared for parts of the United Kingdom on Sunday as an unusual spell of May heat intensifies. Saturday saw a temperature of 30.5C recorded at Frittenden in Kent, marking the highest temperature of the year so far.

The soaring temperatures, forecast to build further across England and Wales on bank holiday Monday, could also lead to new national May temperature records being set. The current highest temperature recorded in May is 32.8C, set in 1944. Current forecasts indicate the possibility of exceeding 33C in parts of south-east England.

For some areas, it is likely that Sunday's confirmed temperatures will meet the criteria for an official heatwave, which requires specific temperature thresholds to be reached or exceeded on at least three consecutive days. These thresholds vary by region, ranging from 25C in Scotland and northern England to 28C in London and surrounding areas.

The Met Office's criteria for a heatwave depends on local climate norms, with thresholds set at 25C for Scotland, Northern Ireland, much of Wales, and northern England. For many other parts of England and south-east Wales, it is 26C or 27C, while London and some home counties require 28C for three consecutive days.

Beyond daily temperatures, the night-time temperature record for May could also be broken. The current record stands at 18.9C, set in Folkestone on the night of May 30-31, 1947. If any location remains above this temperature overnight, a new record could be established.

The longevity of the heatwave is a concern, with amber heat-health alerts remaining in force for parts of the Midlands and eastern England until at least Wednesday. While the tourism and hospitality sectors may welcome the hot and sunny weather, impacts on public health are anticipated.

Climate change is believed to be a contributing factor to such hot spells. Data from the Met Office indicates a significant increase in days with high temperatures, with those exceeding 28C more than doubling and days above 30C more than trebling in the most recent decade compared to the 1961-1990 average.

However, the sunshine and heat will not be universal across the UK. Parts of north-west Scotland are expected to experience cloudier conditions at times, with a chance of occasional rain and cooler temperatures.