Britain could turn tropical tomorrow with high humidity and the hottest day of the year so far, before "violent" thunderstorms bring a dramatic end to the heatwave, forecasters say.

The UK has seen its longest prolonged heatwave in seven years, although temperatures dipped slightly over the weekend.

But tomorrow, the mercury is expected to reach 33C (91.4F), with the Midlands and the south of England the likely contenders for the hot spots of the day.

The hottest day of the year so far had been last Wednesday at Hampton waterworks, south west London, with highs of 32.2C (90F).

Brendan Jones, a senior forecaster with Meteogroup, the weather division of the Press Association, said: "It could well get over that tomorrow. The air has been pretty dry so far in this heatwave so it's not been overly humid.

"Monday will be very, very humid, with the air coming up from France and Spain for the first time in this hot spell.

"So while 33C would be about a degree higher than the hottest temperature so far, it will feel even warmer. It's going to be sticky, oppressive and close, and will make things feel quite uncomfortable."

There could be isolated thunderstorms in London, the south of England and parts of south Wales overnight tonight, with some rain tomorrow.

People should make the most of the sunshine however, with cooler weather on the way, and potentially heavy rain on Tuesday.

The Met Office has issued a rain warning for Tuesday for most of England and all of Wales, with localised flooding possible in places.

"We could have a few fat spots of rain coming out of the sky over widespread areas on Monday" said Mr Jones. "With the high temperatures, it will be almost tropical in the way it feels tomorrow.

"Then from Monday night and into Tuesday, some very violent weather could be kicking off, with thunderstorms across quite a lot of England, Scotland and Wales. We will see quite lively storms.

"It will stay pretty warm as well, with temperatures up to 30C (86F) on Tuesday, and some humidity."

Temperatures will start gradually dropping as the week continues but will remain in the mid 20s, which is still over the average for this time of year, Mr Jones said.

"It's the end of the official heatwave. But while they can be broken down very quickly and turn cold overnight, it's still looking quite warm next week," he added.

The hot weather has taken its toll on the UK in recent weeks, with grass fires in London, mountain blazes in the Welsh valleys and forest fires in Fife, Scotland. Hundreds of premature deaths are believed to have been caused by the heatwave.