Bonneville placed a message on his official website where he wrote: "To clarify recent speculation, the Labrador that appeared in Series One (1912-14) was a dog called Pharaoh.
"From Series Two (1916-1920) onwards, the Labrador has been a bitch named - in keeping with the Egyptian theme - Isis.
"Anyone who genuinely believes the Series 5 storyline (1924) involving the animal was a reaction to recent world news is a complete berk."
Meanwhile, ITV has just announced it is commissioning a sixth series of the show.
The fifth season comes to an end on Sunday and has been watched by an average audience of more than 10 million viewers.
Producer Gareth Neame said: "We have had an amazing reaction to the storylines, acting and production values of Downton Abbey this year and the whole team is thrilled to be moving ahead with a new season of the show. This will provide audiences with a fresh opportunity to see what will befall our much-loved characters."
Production will begin on the new series early next year.
- PAA