Homeowners in the Watford area have been put on red alert amid fears that further flooding could spark a deluge of rats.
More than 7,500 homes and businesses in the UK have been badly affected by floodwater since the beginning of December.
And more communities are in danger over the next few days after forecasts of fresh heavy rain lasting into next week.
There has already been serious flooding in Lower High Street and Water lane in Watford, in Green Street in Chorleywood, in Moor Lane, Rickmansworth and in London Colney.
The continued deluge has prompted a warning from a professional pest control registry, which says rats will try to escape rising water levels.
Rob Simpson, managing director of Basis Prompt, said: “Infestations of rats often increase during very wet weather as flooding will flush them out and force them to find new home.
“They will try to find some sort of dwelling to live in – it can be garages or sheds or even lofts to get away from the water and the knock-on effect is more infestations in homes.
“Rats are intelligent creatures able to adapt and replenish their numbers very quickly. They are always on the look-out for new opportunities.”