The United Kingdom Met Office issued two "red" weather warnings as hundreds of residents across northern England were evacuated from their homes and the army was deployed to build flood defenses.
The worst affected area was northwest England, where residents already hit by severe floods in recent weeks faced more heavy and persistent rain.
The Met Office's most serious weather warning was issued for Lancashire on Saturday with a risk of "widespread river flooding." It later issued a similar warning for west and north Yorkshire.
Twenty severe flood warnings - indicating a danger to life - have been issued by the Environment Agency, with an additional 218 flood warnings and 132 alerts, mainly for the northwest, northeast, and Wales.
The severe flood warnings - the agency's highest level - are for locations on the Ribble, Calder, and Aire Rivers.
"Very severe weather conditions are expected," the Met Office said in an advisory on its website. "Widespread flooding will lead to severe disruption to travel and danger to life."
Alan Tomlinson, who lives in the village of Ribchester, in Lancashire, described the situation as "pretty grim."
"The village is under siege really," he told the British Broadcasting Corp. "The routes to the south are cut off and the route to the north, to Longridge, is under threat."
Kellie Hughes, from nearby Whalley, said the situation was "a million times worse" than flooding earlier this month.
"It's just horrific, really bad," she told the BBC. "I've got the sandbags down here and just doing the best I possibly can. There are no more sandbags anywhere. People are panicking."
The British government's emergency Cobra committee met for a second consecutive day Saturday, led by Environment Secretary Elizabeth Truss.
"Severe flood warnings were issued early this morning by the Environment Agency, due to the impact of further rain on already saturated ground, in Lancashire," Truss said in a statement. "There are also ongoing concerns around possible impacts in Yorkshire which we are monitoring closely."
Government minister Rory Stewart said some flood-hit areas could have a month's rainfall in a day on ground that's already saturated.
"What we've seen is rainfall levels that nobody's ever seen before," Stewart told the BBC Radio 4 Today program. "If somebody had said two years ago when we were designing these flood defenses that we could get 13 inches of rain in a day, the answer from the engineers would have been 'Why are you making that kind of prediction? We have never seen this before.' "