Forecasters warn of washout Tuesday
Half a month's rain could fall in a single day in parts of the country this week.
The South West and Wales are likely to be the worst affected, with up to 25mm on Tuesday.
Towards the end of the week there will be spells of sunshine and showers, with temperatures climbing to 15C (59F) or 16C (61F) in London on Thursday and Friday.
The maximum for Tuesday in London will be 12C (54F).
Matt Dobson, a forecaster with MeteoGroup, the weather division of the Press Association, said towards the end of May we would see a return to more "settled" weather.
He said: "There is some rain on the way. Tuesday is looking like the wettest day of the week for southern parts of the country.
"There will be lots of rain moving from Devon and Cornwall and the South West to Wales, with up to an inch or 25mm on Tuesday daytime. It's a lot for a day - a typical place in southern Britain would see around two inches of rain in a whole month. Places could see half a month's rain in a day."
But he said that predictions of a summer as wet as last year were premature.
"Time and time again it's been shown these kinds of long range forecasts don't work", he said.
"Looking forward though there are signs of slightly more settled weather towards the end of May. It's still developmental work trying to predict a few months ahead."