Chocolate giant Cadbury and the National Trust are running a spring event at around 300 of the conservation charity’s properties called the Cadbury Egg Hunt.
The word Easter is mentioned several times on the National Trust webpage promoting the hunts, but does not feature in the event logo.
The prime minister has joined the Archbishop of York in condemning the move.
Mrs May, who is a vicar’s daughter and a member of the National Trust, told ITV News: “I think the stance they’ve taken is absolutely ridiculous and I don’t know what they’re thinking about.
“Easter’s very important. It’s important to me, it’s a very important festival for the Christian faith for millions across the world.