Householders and business owners have been warned to be prepared for further flooding, as torrential rain sweeps across the country.

The Met Office issued an amber alert for the east, south-east and south-west of England and there were concerns that more rain in mid and west Wales could cause further damage.

By lunchtime on Monday the Environment Agency had issued 35 flood alerts – signalling that flooding is possible – for parts of the south and north of England, the Midlands and Wales, after up to 50mm of rain fell in a few hours.

One flood warning – meaning flooding was expected and immediate action would be needed – was issued for areas of Surrey and West Sussex around Gatwick airport. The airport said services were working normally.

Firefighters in West Sussex received more than 80 flood-related calls between 10.30pm on Sunday and 3am on Monday. Some homes in Littlehampton were under 1.2 metres of water, and a temporary centre for evacuees was set up at a leisure centre. About 40 properties were affected.

A fire service spokesman said: "There are no reports of any injuries, but people are being urged to stay out of floodwater due to the dangers of lifted manhole covers and water contaminated with sewage."

Some schools on the south coast were closed and health emergency cases were diverted from Worthing hospital in West Sussex to a nearby centre. There were road closures and some traffic disruption throughout the south-east.

The weather is proving a bonus for pest control firms, who are reporting an increase in callouts to rat infestations. Rats are thought to be moving close to the surface as they flee flooded sewers.

In the Welsh village of Pennal in Gwynedd, between Machynlleth and Aberdovey, a landslide blocked a channel through which water normally drains from a disused quarry. The heavy rain meant the quarry was in danger of filling and the water spilling into the village below.

North Wales police told people to leave their homes and most of the 600 residents moved in with friends or relatives or into the leisure centre at Machynlleth, where a temporary refuge had been set up. Emergency services managed to cut a new channel from the quarry to allow water to seep out and residents were told it was safe to return.

A fire service spokesman said: "The pressure on the reservoir was immense, so public safety was paramount. We have managed to avert a very serious situation."

Hundreds of residents and holidaymakers returned to homes and caravans in and around Aberystwyth which had been hit by floodwater up to 1.5 metres high. Firefighters were continuing to pump out water. About 150 people had been rescued or evacuated after 150mm of rain fell in 24 hours, twice as much as normally falls in the whole of June.

Two teenagers were winched to safety after a static caravan where they were staying was surrounded by floodwater. Emily Nickless, 18, and Leigh-Anne Wharton, 17, were asleep in the caravan at the Riverside park in Llandre, near Aberystwyth, when Emily's grandfather, Sam Nickless, spotted it on TV coverage of the floods and rang to warn them they were surrounded by water.

They called for help and were rescued by an RAF helicopter crew.

Emily said: "I got woken up by a call on my mobile. It was my granddad, he told me to look out the window because the campsite was on the news because it was under water. When we opened the door, it was terrible. It was impossible to get out."

The Welsh first minister, Carwyn Jones, said the government was spending £40m on bolstering flood defences and tackling coastal erosion over the next year. "The reality is you can't prevent flooding at all times, especially when you get very unusual weather patterns such as we've seen over the last few days in this part of Wales. The situation will be examined, we'll talk to the Environment Agency about what could be done to help boost flood defences in the future," he said.

Health officials in Wales warned people to stay away from the floodwater and to seek medical advice if they had accidentally swallowed any and felt ill.

In Waterlooville, Hampshire, a man had to be rescued by firefighters after his car became stuck in a deep water in a ford and started taking in water running off from nearby fields. The water had reached seat-level by the time the man was helped to safety.

Mick Crennell, of Hampshire fire and rescue, said: "This incident highlights the dangers of attempting to drive through deep water and it gives an indication of how quickly the situation can change and become life-threatening."

Parts of the M1 in West Yorkshire were shut during the night after flash-flooding made driving hazardous, but the road was reopened by Monday morning.