Parts of Britain are set to bask in the hottest temperatures of the year on Wednesday, but forecasters have warned the unseasonably warm weather will soon be brought to an abrupt end.

The mercury is expected to reach as high as 22C (71F) in London, South East England and East Anglia - matching this year's highest temperatures measured in Gravesend and Manston in Kent on April 14.

But weather experts have predicted temperatures will plummet across the country by Friday as a cold front sweeps much of the UK.

Laura Caldwell, a forecaster for MeteoGroup, the weather division of the Press Association, said: "It will be quite a warm day in the South East, although it won't be as sunny as today. We could hit 22C (71F) in London, Kent and East Anglia.

"Elsewhere, we're in for a bit of a mixed bag. They'll be rain across northern and central parts of the UK, with some sunny spells in Scotland.

"The rest of the country will have double digit temperatures, with Scotland and Northern Ireland at about 10C (50F) or 11C (51.8F)."

Ms Caldwell said temperatures would likely be "pulled down" by Friday, with average temperatures between 9C (48.2F) and 12C (53.6F).

"A cold front will bring rain down across the UK towards the end of the week," she said.

"Scotland will be quite colder on Thursday and temperatures will slide across the UK from Friday. Maximum temperatures in the South East will reach 12C (53.6F)."

Meanwhile, hay fever sufferers received some good news as experts predicted a shorter hay fever season this summer. Cold weather over recent months is thought to have delayed the release of silver birch and grass pollen.