The estranged wife of Sir Paul McCartney is backing a campaign by animal welfare group Viva! to ban farrowing crates from British farms.
Viva! activists claim that sows are "held captive" inside the crates for a month every time they become pregnant.
According to reports, Mills joined Viva! activists on a raid of Briarwood Farm in Greinton to film conditions inside the farrowing crates. The video reportedly shows Mills standing next to a breeding sow while she criticises the conditions inside the crates.
Michael Underhill, the owner of the farm, is reportedly planning to take legal action against Mills.
He said: "This is trespass. We can't just have people coming on to the farm whenever they choose."