The 500lb Bengal tiger ripped off Nigel Wesson's left arm at the elbow after the keeper put his hand back into the cage to close a partition.
A spokesman for Chipperfields Circus said Mr Wesson broke an "unwritten rule" by putting his arm into the animal's cage.
And he said Rajah would not be destroyed despite the horrific accident - the second to hit the family this year.
Mr Wesson, 32, who lives in a caravan next to the circus's winter quarters in Green Lane, Heythrop, near Chipping Norton, was airlifted to the John Radcliffe Hospital at Oxford.
His arm was amputated above the elbow and he is today in a stable condition.
Mike Tunnicliffe, spokesman for the
family and brother-in-law of 24-year-old Richard Chipperfield - who suffered appalling brain injuries in a mauling in Florida last month - said Mr Wesson was an experienced keeper who joined from another circus just over a fortnight ago.
Two or three other members of staff were nearby when the accident happened. One administered first aid.
But Mr Tunnicliffe added: "It is an unwritten rule that you do not put your hand into a cage. Mr Wesson seems to have lost concentration for that second and we are all extremely sorry.
"Mr Wesson is a very experienced, methodical person. The accident happened in seconds and we are heartbroken about it."
This morning Rajah was playing with his two sisters in an open-air compound.
Mr Tunnicliffe said: "There is no question of putting the tiger down and I am sure Nigel would agree. Rajah does not know he has done anything wrong."
Rajah, four, was bred by the Chipperfields and appeared in Esso TV advertisements.
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