Thendara Satisfaction, known as Jagger, is understood to have collapsed and died after returning home to Belgium at the weekend, BBC reports.
Police in Belgium are investigating and a full toxicology report is expected soon.
Jagger, who came second in his class at the show, at Birmingham's NEC, is owned by Belgian Aleksandra Lauwers and Leicester-based breeder Dee Milligan-Bott.
Mrs Lauwers's husband Willem said the couple was "devastated" by the death.
He said CCTV from the NEC was being analysed, but the number of dogs and members of public at the event could make it very difficult.
Mrs Milligan-Bott said she could not believe another exhibitor was responsible.
"We compete week-in, week-out against each other and we have one thing in common, we all love dogs," she said.
"I think and hope it was a random act by someone who hates dogs, an opportunist.''
She said a post mortem showed Jagger had been poisoned and a full toxicology report was expected to reveal more.
The Kennel Club, which organises Crufts, said it was unclear what had happened but it would support the police in their investigation.
