They are believed to have fallen into the water after playing on ice in Babbs Mill Park, Kingshurst. They were taken to two hospitals in Birmingham.

West Midlands Ambulance Service said the children had been in cardiac arrest when they were rescued, but offered no further update on their condition.

The Met Office has warned of the risks of ice as the UK faces a cold spell.

During a news conference on Sunday evening, West Midlands fire chief Richard Stanton said crews were told on arrival that up to six people had been in the lake.

He said: "We have continued search operations in order to confirm whether there were any more in the water."

But he later clarified that "this would no longer be a search and rescue operation", citing the coldness of the water, the age of those who had entered, and the time they may have been in there.

The ambulance service said during the news conference that its resources had been stood down.

Supt Richard Harris, of West Midlands Police, added that his officers would "hold the scene tonight", but he could not confirm they were looking for anyone else at this stage.

He said one officer had suffered "mild hypothermia" during the rescue operation, but was making a recovery after being taken to hospital "as a precaution".

All three emergency services said they were called shortly after 14:30 GMT on Sunday, with the first ambulance arriving within seven minutes after it was called.

By the time firefighters arrived, both police office officers and members of the public were involved in rescue attempts, Mr Harris said.

Temperatures of 1C (34F) were reported in the area at the time. They are expected to plummet to -3C overnight.

The UK is expected to continue experiencing a cold spell - with snow, ice and freezing fog expected for days, according to forecasts.

Temperatures could drop as low as -15C in northeast Scotland overnight, which would be the lowest temperature recorded in the UK since February last year.

The four children received immediate treatment on site, said Cameron McVittie, a tactical commander with the West Midlands Ambulance Service.

They were then taken under blue lights to hospital, each with an advanced care team, Mr McVittie said - two going to Birmingham Children's Hospital and two to Heartlands Hospital.

He added: "There are no updates. They were all in critical condition when they arrived in hospital."

The officials declined to provide any further information about the children involved in the incident.

Concerned members of the community had gathered near the scene in a residential area, as emergency services worked at the lake.

A drone was also spotted flying over the water.

Babbs Mill Park is a local nature reserve with a lake, river, wildflower grasslands and woodlands.

West Midlands Mayor Andy Street tweeted to say it had been a "deeply concerning incident", adding: "I know we'll all be praying for the children to pull through".

The Met Office warned of the dangers posed by ice during the UK's cold spell.

A spokesperson said: "With temperatures set to struggle to get much above freezing over the coming days across much of the country, stretches of water such as rivers and ponds are at risk of freezing.

"While frozen rivers and ponds can be fascinating, they can be dangerous and a hazard to life."