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The Met Office has issued a fog warning for Kent until 10am this morning.
A spokesman said: "Patchy, thick fog is likely to cause poor driving conditions.
"Anyone travelling should be prepared to expect slower car journeys with possible delays to bus and train services."
He added: "There may be delays or cancellations to flights."
Although not all places are affected, there are areas of fog with visibility below 100m, particularly in river valleys and along the coast.
He warned: "Be prepared for sudden deterioration in visibility over a short distance."
Police and driving organisations repeated advice for motorists to slow down, leave extra space from the vehicle in front and to always drive in fog with at least dipped headlights and to use fog lights front and rear where appropriate.