Forecasters have warned of possible flooding as the UK’s heatwave gives way to heavy downpours and thunderstorms.

The Met Office has issued weather warnings covering almost the whole country from Monday morning, identifying there is “a small chance that homes and businesses could be flooded quickly” following rain showers, with the chance of “significant and disruptive” thunderstorms.

The far north of Scotland is the only part of the UK not currently covered by Monday’s weather warning, which is in place across the whole of England and Wales until at least Thursday.

Northern Ireland is affected from Monday until Wednesday, although forecasters expect Devon and Cornwall to be the first to experience thunderstorms this week.

It follows an intense period of very hot weather which saw temperatures reach 34C (93.2F) at Herstmonceux in East Sussex on Sunday, the fourth consecutive day thermometers in the South of England passed 30C (86F).

Met Office forecaster Craig Snell said: “For England and Wales over the next four days, there is a risk of some very severe weather in places but it is not going to be everywhere.

“Away from the thunderstorms, it is still going to be very hot.

“It is not going to be wall-to-wall sunshine, but it is not going to be a washout each day.”

The warning means there is a risk of torrential rain, hail, frequent lightning and strong gusty winds in much of England and Wales if intense thunderstorms break out.

Downpours could see 0.8in-1.2in (20mm-30mm) of rain fall in an hour, with some locations potentially receiving 1.6in-2.4in (40mm-60mm) in three hours.

The Met Office said these would be fairly isolated incidents, and the Environment Agency did not have any flood alerts or warnings in place as of 9am on Monday.

Regarding Monday, Mr Snell said: “The warning does not imply that everyone is going to see them (thunderstorms) – a lot of places will remain dry, sunny and hot – but we will see some fairly severe thunderstorms developing over the course of the day.”

The Met Office warned that flash flooding could cause travel disruption and power cuts, but also cautioned about the risks caused by fast flowing or deep floodwater.

It came as a 12-year-old girl died after going missing in the River Leven, near Ballock Bridge, Loch Lomond, in the west of Scotland on Sunday evening. Her body was discovered by emergency services hours later.

In Norfolk, a woman in her 30s died after getting into difficulties in the sea at Waxham on Sunday.

HM Coastguard dealt with 340 incidents across the whole of the UK on Saturday – the highest number of call-outs in a single day for well over four years.

In Dorset, traffic police said a colleague stopped a father with four children arriving near coastal beauty spot Durdle Door, only to find it had closed.

The group had travelled down from Birmingham and spent more than six hours in their vehicle only to be turned back, police said.