Opening Prime Minister’s Questions (PMQs), Mr Johnson said: “Our sympathies also go out to those affected by the latest floods.
“I want to thank the Environment Agency and our emergency services for the work they’re doing to support those communities, and I’ll be chairing a Cobra meeting later on to co-ordinate the national response.”
It comes after Flooding Minister Rebecca Pow convened the emergency committee yesterday afternoon in response to Storm Christoph.
Major incidents have already been declared in South Yorkshire and Greater Manchester amid amber and yellow weather warnings for the storm, which could also bring snow to northern areas.
More than 120mm of rain has already fallen in parts of the country, with 123.4mm at Honister Pass in Cumbria in the 24 hours up to 6am today.
Nearby Seathwaite saw the second highest total, with 107.2mm, and some isolated spots could see up to 200mm, the Met Office said.
Almost the whole of England, Wales, and Northern Ireland are subject to yellow weather warnings for rain until midday on Thursday, with a more serious amber warning stretching from the East Midlands to the Lake District.
The amber alert warns of the risk of flooding and deep floodwaters which could pose a risk to life, and there are further yellow warnings for snow and ice in Scotland.
Met Office forecaster Grahame Madge described Christoph as “quite a slow-moving system” which is bringing “a variety of weather” to the UK.
The meteorologist said: “While rain remains the main hazard in the south, further north we’ve got snow and ice remaining a risk.
“The system will work its way through, we are expecting significant totals of rainfall and when you combine that with snowmelt it can lead to localised flooding across the affected regions.”
Fifty flood warnings have been issued across England, with 175 less severe flood alerts.
People were also preparing for rising floodwaters in West Yorkshire this morning, with towns in the Calder Valley laying out sandbags and flood wardens monitoring water levels.
As the week continues and Storm Christoph makes its way east, the risks of snow increase, Mr Madge said.
“When Christoph starts to move into the North Sea we will start to see more northerly influence coming in, we’re going to have a feed of colder air which will bring in wintry showers,” he said.
“Over higher ground to begin with but increasingly those events will be to lower levels as well and that could lead to some accumulations in Scotland, northern England and parts of Northern Ireland.”