South Wales Police say a man has been airlifted to hospital after being injured at the Tata Steel plant in Port Talbot.
Officers received the call of a 'serious incident' at 16.51 today.
The man, a worker at the site, is currently at the University Hospital of Wales, where his condition is described as critical.
More top news
There will be a fresher feel with temperatures falling lower than on recent nights.
The accident between junction 24, Coldra, and 23A, Magor, has closed the eastbound carriageway.
Eloise Parry, 21, died after taking diet pills containing the poisonous Dinitrophenol (DNP).