Cameron sent out a Twitter message describing the "shocking scenes of flooding in Cornwall and around the country" during a visit to the hard-hit region this weekend.
The BBC said Cameron pledged the government would "help ensure everything is being done to help."
Two people were killed in the storms, which also flooded hundreds of homes and were forecast to roll through southwestern England and Scotland on Sunday. Transportation was also a mess, with roads flooded and trains delayed.
Government officials warned rural residents that saturated ground could lead to further flooding on short notice in the coming days.