The risk of an explosion at a large warehouse fire closed both carriageways of the M25 between junctions 22 and 21A.
Hertfordshire Fire and Rescue was called to the blaze in Ventura Park, Old Parkbury Lane, Colney Street, in St Albans, at 09:21 BST.
National Highways said 15 crews were at the scene and the motorway had been closed because of the a risk of a “secondary explosion, along with smoke and debris affecting the carriageway”.
The road reopened with 40mph speed restrictions in place at about 12:15.
People were asked to keep their doors and windows closed and to avoid the area.
Hertfordshire Police confirmed the M25 had closed due “to smoke from a warehouse fire”.
It said there were also road closures in St Albans, at Colney Street and Old Parkbury Lane.
National Highways said because the warehouse was close to the railway line, Network Rail had a representative on site liaising with the fire service.
There were still delays in the area and it might “take time for congestion to clear”, it added.
The fire service said it had “received a significant number of calls regarding this incident”.
A spokesperson added that firefighters had been “working hard to bring the situation under control”.
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