The former Blue Peter host and Doctor Who star said he was told the broadcaster is "refreshing" its line-up.
The 80-year-old told ITV's Good Morning Britain: "I got a phone call saying 'There is no easy way to say this but actually you're surplus to requirements, we don't need you'.
"I was stunned actually, there hadn't been a hint about it, not a hint last year at the show. This would be my 42nd year at the show. I just feel it's part of my life, so I was very shocked, I was very hurt."
Purves has served as a presenter and commentator for the dog show for 41 years.
He said he has instead been invited to be a paid guest contributor, adding: "The point was I'm not part of the commentary team and the executive producer has invited me to be a guest contributor on two of the programmes on the Saturday and Sunday.
"I'm going to be at the show because I'm gong to be presenting the award so it's no skin off their nose so they can do something very nice for me.
"I feel it was just a bit of a sop and it was perhaps to soften the blow and it didn't."
He added: "It's been part of our lives. It's a four-day job but at my age there isn't a lot of television work for people my age.
"I did ask for the reasons to be in writing. I got them. There wasn't a reason, it was just 'We are always refreshing the show, sorry, one in, one out."
Asked if he thought "refreshing" was code for ageism, he said: "I believe the diktat from Channel 4 was we want a younger presentation team. I know that was said."
A spokesman for the production company that makes Crufts, Sunset+ Vine, told the PA news agency: "Peter has been a much loved part of the Crufts team and we're delighted he will continue to be part of our coverage and join us on-screen at the weekend of Crufts, the culmination and most watched part of the four-day event."