The viral, which can be viewed , promotes the Opel Astra and uses the strapline "men's new best friend". It has been hosted on a number of sites, starting at the beginning of August.
The ad was created by McCann Erickson, according to the Financial Times, at the same time as the agency was shooting a television commercial for the new Opel Astra GTC.
The RSPCA has criticised the ad and said that it plans to approach advertising regulators about the spot.
The protest follows the outcry over a viral campaign for Ford's SportKa last year, which showed a cat being decapitated by a sunroof. The ad was created by Ogilvy & Mather, and Ford said that it had not authorised its release.
An earlier spot for the same model showed a car bonnet flying up and killing a pigeon that had just relieved itself on its shiny exterior. That ad had pigeon fanciers up in arms.
Unlike Ford, which reacted by donating the use of a van to the Cats Protection League for six months, General Motors is standing by its ad.
"No animal was hurt in the making of this ad. We certainly did not set out to offend anyone," the company said.
The RSPCA said: "Though trying to be clever, this is sick advertising that tries to use a faithful pet as a throw-away gimmick. It sends a brutal, couldn't-care-less message to sell a car. Just last month, the RSPCA heard of a case in Middlewich, Cheshire where three kittens were thrown out of a car window and died.
"The RSPCA will be writing to the Advertising Standards Authority and Vauxhall Motors about the ad. We hope anybody considering buying Opel-Vauxall car products will do so too."
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