Announcing the critical incident, South Wales Police said: “Following flooding and severe weather related incidents which have occurred overnight and through the course of the morning, a critical incident has been declared.
“The emergency services, jointly with local authority planning departments, local health boards, and organisations such as Natural Resources Wales and utility companies, are working continuously to ensure the safety and welfare of those affected, minimise damage to infrastructure and property, and minimise disruption.”
It came after South Wales Fire and Rescue Service received 72 calls between 6pm on Friday and 6am on Saturday.
Pontypridd, which was flooded two weeks ago, and the Ely area of Cardiff are particularly affected, police said, as they warned residents to stay indoors unless “absolutely necessary”.
More than 600 homes and a similar number of businesses have been hit in Wales, accounting for around a quarter of affected properties in the UK.
Superintendent Andy Kingdom said: “Indications are that the rain is set to stop and river levels will soon recede.
“There is still significant surface water and debris on the roads throughout the force area so we would advise people only to travel if absolutely necessary.”
England has had more than 200 per cent of its average February rainfall, according to the Environment Agency, with some areas getting a month’s worth in 24 hours.
Storm Jorge – this month’s third named storm – has prompted weather warnings for rain, strong winds and snow stretching from Cornwall to the north of Scotland and across to Northern Ireland.
A total of 81 flood warnings are also in place across England and Wales, mostly in the South West and along the English-Welsh border, and in Yorkshire, while a further 213 “flooding is possible” alerts are in force.
Met Office forecaster Emma Salter said rain is expected to continue until about 11am, before sunshine dominates through the middle of the day.
Snow will continue to fall, especially in the Scottish Highlands, where Ms Salter said up to 30cm is predicted in some places.
The wind is also expected to be a factor, with gusts up to 75mph expected in more exposed places in northern England and the Highlands, she said.
Heavy rain which started with Storm Ciara, continued with Storm Dennis and now Jorge has contributed to record river levels which have seen safety teams put in “Herculean efforts” to erect flood defences.
Towns including Ironbridge and Bewdley along the River Severn in the West Midlands, and West Cowick and Lidgate in East Yorkshire, along the River Aire, are among the worst-hit areas in England.
The Environment Agency said 1,000 staff per day have worked on flood defences and pumps, clearing debris and repairing damaged defences, erecting 3.7 miles of barriers.
The body warned the country needs to brace itself for “more frequent periods of extreme weather like this” because of climate change.
Ms Salter said the Met Office encouraged people to “slow down, plan ahead and look out for each other” during the cold and stormy weather.
Storm Jorge has been named by the Spanish Meteorological Service, which is part of the southwest Europe storm naming group.
Storm Ellen was the next named storm on the Met Office’s list but the Spanish meteorological service got their name in first when they named the weather system sweeping in off the Atlantic on Thursday.
It is convention for all other national meteorological services to then use that name when referring to the low pressure.
Additional reporting by Press Association.