Scientist Richard Dawkins has apologised for the "feeding frenzy" which was triggered by his tweet claiming it would be "immoral" to carry on with a pregnancy if the mother knew the foetus had Down's Syndrome.
His latest Twitter row erupted after he responded to another site user who said they would be faced with "a real ethical dilemma" if they became pregnant with a Down's Syndrome baby.
Professor Dawkins tweeted: "Abort it and try again. It would be immoral to bring it into the world if you have the choice."
In a fuller explanation on his website - entitled Abortion & Down Syndrome: Apology for Letting Slip the Dogs of Twitterwar - the author tried to set the record straight.
He wrote: "To conclude, what I was saying simply follows logically from the ordinary pro-choice stance that most us, I presume, espouse. My phraseology may have been tactlessly vulnerable to misunderstanding, but I can't help feeling that at least half the problem lies in a wanton eagerness to misunderstand."
The backlash for his comment had included one mother, who has a child with the genetic condition, saying: "I would fight til my last breath for the life of my son. No dilemma" while Prof Dawkins said accusations of "Nazism, vile, monstrous fascistic callousness" and "fireballs of hatred" had been hurled his way.
He wrote "if your morality is based, as mine is, on a desire to increase the sum of happiness and reduce suffering, the decision to deliberately give birth to a Down baby, when you have the choice to abort it early in the pregnancy, might actually be immoral from the point of view of the child's own welfare."
Prof Dawkins claimed he had hoped that his million-plus followers would not see his comments, instead it would only be sent out to the people who follow both himself and the woman who posed the question. He also claimed there was not enough space in his Twitter reply to get his fuller argument across.
He added: "Those who thought I was bossily telling a woman what to do rather than let her choose. Of course this was absolutely not my intention and I apologise if brevity made it look that way. My true intention was, as stated at length above, simply to say what I personally would do, based upon my own assessment of the pragmatics of the case, and my own moral philosophy which in turn is based on a desire to increase happiness and reduce suffering."
He also argued: "Those who took offence because they know and love a person with Down Syndrome, and who thought I was saying that their loved one had no right to exist. I have sympathy for this emotional point, but it is an emotional one not a logical one. It is one of a common family of errors, one that frequently arises in the abortion debate."
Some Twitter users had supported the God Delusion author, agreeing with his assertion that there is a difference in deciding on a termination before a child is born, and suggesting after the child is born that it should have been aborted.
The Down's Syndrome Association (DSA) issued a response to Prof Dawkin's initial comment.
They said: "People with Down's Syndrome can and do live full and rewarding lives, they also make a valuable contribution to our society.
"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."
Previously Prof Dawkins caused controversy on Twitter when he said the world's Muslims had won fewer Nobel Prizes than Trinity College Cambridge.