Firefighters have been trying to get a blaze under control at a recycling centre since Saturday night.
Crews from Hemel Hempstead, Watford and Wheathampstead have been using water jets and hose reels to douse the massive pile of recycled wood at the centre at Appspond Lane on the border of Hemel Hempstead and St Albans.
Herts Fire Service spokesman David Henning said: “There was quite a considerable amount of recycled wood so there’s plenty of scope for it to burn at a considerable pace.”
The alert was raised at 11.45pm on Saturday and a number of fire crews have been working to keep the blaze contained.
The A414 and the A405 have been closed in both directions since 3am this morning (Monday) between King Harry Lane in St Albans and Breakspear Lane in Hemel Hempstead. The roads could be closed for the next two days because of smoke.
A diversion from the A414 St Albans Road directs traffic to the A41 Watford Road, A41 Gadeside and A405 North Orbital Road. Motorists have been advised to expect heavy traffic in St Albans and Hemel Hempstead.