The warning came as it emerged that some of the highest tides of the year will coincide with the severe weather already predicted.
Forecasters said on Thursday that heavy rains and winds of up to 80mph will lash large parts of the country on Monday.
Yesterday, they also warned Britons to stay indoors tomorrow night when the gales are expected to begin and to avoid unnecessary journeys.
The unsettled weather is expected to climax on Monday morning around rush hour when the deepening low-pressure system swings east across the UK.
The Met Office has issued severe weather warnings for much of the UK on Monday.
Brian Golding, head of forecasting, said: "The arrival of this exceptionally deep area of low pressure is expected to coincide with spring tides on Monday, bringing the risk of flooding to western coasts from southern England to northern Scotland."
The storm also brings the risk of disruption to transport and power networks, with winds strong enough to uproot trees and damage buildings, said forecasters.
They warned it "could well be the strongest storm of the winter" but at this stage they are not predicting it will be as severe as that of October 1987.