Warning over travel in Beds, Bucks and Herts
Road users across the East of England are being advised to check the latest weather forecast and road conditions before they travel.
The Highways Agency said it was expecting weather conditions on Sunday to become increasingly more difficult.
Snow is falling in Buckinghamshire, Bedfordshire and Hertfordshire.
"Road users are advised to take extra care when travelling due to the increased risk of adverse driving conditions," the agency said.
"Drivers are also advised to plan their journey before they set out, to check the forecast and road conditions."
Where conditions prove to be poor and a journey is unavoidable, drivers are asked to leave extra time or to consider delaying their journey if the weather becomes severe.
The Highways Agency is working closely with the Met Office to monitor weather conditions.
First Capital Connect operating trains between London and Norfolk through Hertfordshire said severe weather combined with engineering works brought changes to services.
Buses are replacing trains between King's Lynn and Ely and on Hertford services.
"This means journeys will be extended and may be disrupted by the cold conditions," a spokesman said.
"Please check for engineering work, allow extra time and check our live train updates page before you travel.
"Emergency engineering works are causing disruption between Ashwell & Morden and Baldock.
"Because of this, buses are replacing trains between Royston and Hitchin. This will continue until further notice," the spokesman said on Sunday evening.
Greater Anglia said it would run planned services on all routes.
"Scheduled engineering works will be taking place this weekend on some routes and customers are strongly advised to check their planned journey before setting out," a spokesman said.
"Any train service alterations that may be necessary for the morning peak on Monday 21 January, will also be published on this website."
Problems with overhead power cables in Tottenham Hale had caused some delays and cancellations on the line from Hertford into London but engineers anticipated this would be repaired and the route back to normal by 01:30 GMT on Monday.