The Met Office issued a severe weather warning lasting to around midday, and said reduced visibility would continue to affect southern England and south Wales creeping into the Midlands.
Meteorologist Steven Keate
s said visibility and conditions are improving but warned the Met Office may issue another fog warning for Sunday night.
On Sunday morning he said visibility at Bristol Airport and Exeter International Airport was around 200 metres, around 3,500 metres at Gatwick, 3,000 metres at Heathrow while at London Stansted it was 400 metres.
There is still some dense patches of fog around, but things are improving, he added.
On the whole it is a better picture than it was two or three hours ago. Hopefully this afternoon we will continue to see a little bit more of an improvement.
It could remain a bit grey and murky, but it terms of disruptive fog hopefully that will become a bit less of an issue.
Heathrow Airport warned foggy conditions were affecting flights and advised passengers to check status updates with airlines before heading to their terminals.
A spokesman said: If their flights are cancelled then they should not travel to the airport and should contact their airline to rebook.
With fog forecast to be in the area around London City Airport until lunchtime, when their first flights are scheduled to leave, a spokesman said delays and cancellations are possible.
He added: Therefore we would advise that passengers check with their airline before they travel.
Mr Keates said that heading into Sunday evening and overnight it is quite likely the fog will reform and potentially become quite dense in similar places.
We are considering whether a yellow warning is required again, he added.