Richard Dawkins, the atheist writer, has claimed it is "immoral" to allow unborn babies with Down's syndrome to live.
The professor posted a message on Twitter saying would-be parents who learn their child has the condition have an ethical responsibility to "abort it and try again".
His comments were dismissed by charities and prompted fury online from opponents but he insisted his stance was "very civilised" because foetuses do not have "human feelings".
@InYourFaceNYer Abort it and try again. It would be immoral to bring it into the world if you have the choice.— Richard Dawkins (@RichardDawkins) August 20, 2014
He claimed that the important question in the abortion debate is not "is it 'human'?" but "can it suffer?" and insisted that people have no right to object to abortion if they eat meat.
Anti-abortion campaigners describe the practice of aborting foetuses on physical grounds as a form of "eugenics".
But Prof Dawkins strongly defended its as simply standard practice and ridiculed his critics as portraying him as "a horrid monster".
The row erupted during a debate on Twitter about calls for further changes to Ireland's abortion laws in the wake of the case of a rape victim who was forced to carry the child until she could deliver by caesarean section.
One participant said they would suffer a real ethical dilemma if they were carrying a child with the condition.
Prof Dawkins replied: "Abort it and try again. It would be immoral to bring it into the world if you have the choice."
Another pointed to recent figures asking: "994 human beings with Down's Syndrome deliberately killed before birth in England and Wales in 2012 - is that civilised?"
He responded: "Yes, it is very civilised. These are foetuses, diagnosed before they have human feelings."
He insisted he was not questioning the right of people with Down's syndrome who have already been born to live - just those who have not yet been born.
"There's a profound moral difference between 'This foetus should now be aborted' and 'This person should have been aborted years ago'," he wrote.
And in response to a thread discussing whether animals feel fear, he said: "Precisely my point. Unless you are a vegan (most Pro-"Lifers" are not) you are in no position to object to abortion."
But faced with a stream of dissenting comments, he wrote: "Apparently I'm a horrid monster for recommending what actually happens to the great majority of Down Syndrome foetuses. They are aborted."
Apparently I'm a horrid monster for recommending WHAT ACTUALLY HAPPENS to the great majority of Down Syndrome fetuses. They are aborted.— Richard Dawkins (@RichardDawkins) August 20, 2014
Carol Boys Chief Executive of the Down's syndrome Association dismissed rejected Prof Dawkins's remarks.
"People with Down's syndrome can and do live full and rewarding lives, they also make a valuable contribution to our society," she said.
"At the Down's syndrome Association, we do not believe Down's syndrome in itself should be a reason for termination, however, we realise that families must make their own choice.
"The DSA strives to ensure that all prospective parents are given accurate and up to date information about the condition and what life might be like today for someone with Down's syndrome."
A Scope spokesperson said: 'Finding out you are expecting a disabled child can be a very difficult and confusing time for parents.
"Sadly many tell us they experience negative attitudes. What parents really need at this time is sensitive and thorough advice and information."