Flooding is expected and travel disruption is likely due to difficult weather conditions in southern England.
The Environment Agency had issued 23 flood warnings, meaning flooding is expected, by 3am for an area stretching from Ludgershall in Wiltshire down to Dorset and across to Somerset.
Flights could be cancelled and journeys delayed because of “freezing” fog in the region, the Met Office said.
It issued a yellow warning, to run until 8am, for fog for an area stretching from Exeter to Oxford, with lows in that region of minus 5C recorded at Benson in Oxfordshire, Farnborough in Hampshire and Wiltshire’s MoD Boscombe Down.
South of the Wiltshire military site, at Bournemouth Airport, the overnight low by 3.30am was recorded as minus 6C.
“Freezing fog will lead to difficult driving conditions and could cause travel delays in some areas,” the Met Office said.
Dense fog could see visibility drop to below 100 metres in places, the forecasters added.
Commuters were asked to prepare for longer journey times due to possible delays to bus and train services.
“There is a chance of delays or cancellations to flights,” the Met Office added.
Fog over parts of Dorset, Somerset and Wiltshire will be dense at times overnight but should clear on Wednesday morning, the forecasters said.
Potentially difficult driving conditions were likely due to the combination of reduced visibility and untreated surfaces turning icy.
The Met Office said residents and commuters in Bracknell Forest, Buckinghamshire, Hampshire, Oxfordshire, Reading, Slough, Southampton, Surrey, West Berkshire, West Sussex, Windsor and Maidenhead and Wokingham would need to be mindful of the conditions.
The fog was also due to hit Bath and North East Somerset, Bournemouth, Christchurch, Poole, Devon, Dorset, Somerset, South Gloucestershire, Swindon and Wiltshire.
Unusual weather has “flipped the norm” and left parts of northern Scotland enjoying warmer temperatures than southern England, the Met Office has said.
An overnight low of minus 8.6C was recorded in Benson on Monday, with Altnaharra in northern Scotland a comparatively balmy 10.6C.