A spokesman for West Midlands Fire Service has said the fire at the Smurfit Kappa plant in Birmingham is in "a very large outside storage area, containing cardboard bales".
The site in Mount Street, in the Nechells area of the city, is a recycled packaging mill.
According to its website, Smurfit Kappa is one of the leading producers of paper-based packaging in the world with more than 41,000 employees in 32 countries.
More top news
Cloud and some springtime sunshine
Details of the new arrival were released by Kensington Palace.