Some areas have seen as much rain in the past 48 hours as they usually get in the whole of August.
And the stormy spell that has blighted Britain could be set to last until the final weeks of the month before normal temperatures resume.
The bleak forecast is in stark contrast to the end of July that saw the UK record what is believed to have been its hottest ever day.
And after more days of rain, this month could threaten the wettest ever August of 1912 that saw an average of 167.6mm.
Heavy downpours on Monday will herald the start of an “unsettled” week of wind and rain, according to the Met Office.
While large parts of the country will have a settled day, there is a yellow weather warning in place for thunderstorms across London and the South East on Monday, after a weekend of stormy conditions.
Following Monday’s downpours, temperatures will then struggle to climb out of the teens for the rest of the week as low pressure brings cool, wet weather, according to forecasters.
Bonnie Diamond, meteorologist at the Met Office, said: “This week there’s going to be a fresher feel, winds from the north bringing cooler temperatures than we’ve seen in a while.
“It’s going to be an unsettled week with periods of low pressure bringing rain and wind at times, especially on Wednesday and Friday.
“We’re looking at top temperatures in the high teens.”
Monday’s yellow weather warning, stretching from the English Channel, west towards Portsmouth and north over Essex, brings the possibility of localised flooding and some travel disruption.
Ms Diamond added: “There’s the possibility of up to 40-60mm of rain in just a couple of hours in some places.
“That’s some pretty heavy downpours.”
After a more settled day on Tuesday, the wet and windy weather will then return midweek, with more widespread showers across the UK.
Ms Diamond explained: “Overnight Tuesday into Wednesday, an area of low pressure is bringing a band of rain across the whole country.
“Wednesday is going to be a pretty wet day.”
Over the weekend, travellers across the UK were hit by delays and disruption following heavy rain and strong winds.
Ferry passengers heading into Dover faced delays of five hours as tug boats battled the weather to guide them into port.
Elsewhere, flooded tracks forced train operators to cancel services between Cumbria and Scotland.
Flooding between Penrith and Lockerbie forced operators to suspend services on Saturday afternoon.
Bookmaker Coral has cut the odds on this month ending as the wettest on record to just 1-2 (from 4-5).
“You’d be hard pushed to remember an week in August as wet as we have just had and it is now firmly odds on that this month will break the record and go down as the wettest since records began,” Coral’s Harry Aitkenhead said.