The M11 motorway in Essex has been closed after a lorry overturned and spilled a large amount of acid across both carriageways.
The M11 was shut in both directions between Harlow and Bishop's Stortford near Stansted Airport in Essex at 5.10pm after the heavy goods vehicle turned on to its side.
The spilled chemicals have been identified as phosphoric acid and melamine formaldehyde resin, Essex County Fire and Rescue Service said, and firefighters at the scene are awaiting scientific advice on how best to deal with it.
The Highways Agency says the northbound carriageway is expected to be closed until after midnight while the southbound carriageway could stay shut until 3.30am on Friday.
Further north on the M11, the Agency was reporting delays of two hours between junctions 10 and 8 caused by congestion.
A spokesman for the fire service said he was "not aware" of any danger to the public but the motorway has been closed from junction 7 to 8 and motorists are urged to avoid the area and find alternative routes.
No other vehicles were involved in the incident.
Phosphoric acid is commonly used in fertilisers, food additives and home cleaning products, as well as in medicine and dentistry to help whiten teeth.