Today is expected to be the hottest day of the year so far, with Met Office weather forecasters predicting temperatures could hit 30C. Meteorologist Michael Silverstone said: "If we reach 30C on Thursday, it will be the earliest date in May that the UK has seen 30C since our records began in 1860."

The Met Office said temperatures reached 26.7C in Wisley, Surrey, on Wednesday - making it the warmest day of the year so far. However, the London Fire Brigade (LFB) has issued an urgent warning over a common cold water mistake millions of Britons around the UK enjoying the 'mini-heatwave' could be in danger of making.

The LFB has urged caution around open-water swimming after a 32% increase in water-related incidents last month compared with the same period last year.

Craig Carter, LFB assistant commissioner for prevention and protection, said: "Even when the sun is shining, water temperatures can be dangerously cold. Cold water shock can affect anyone, no matter how fit or experienced they are.

"It can lead to water inhalation and, in the worst cases, drowning. Be particularly careful near the water's edge, it's easy to slip and fall unexpectedly. And think twice before jumping into open water."

Despite the soaring temperatures, the Met Office has ruled out a heatwave this week. Mr Silverstone said it looks "unlikely" the UK will reach a heatwave - defined as three consecutive days of temperatures exceeding the "heatwave threshold", which varies across the country.

The threshold is 25C for most of the UK, rising to 28C in London.

The expert said: "Admittedly, it could be close for a few places in the South, though, with temperatures either today or Friday only just failing to exceed the required threshold.

The warm weather could also challenge the record for the highest April temperature in Wales, which is 26.2C.

Nevertheless, temperatures are expected to ease by Friday, with Saturday bringing cooler conditions of 14C to 18C across the UK.