The Hellenic Olympic Committee said Wednesday that Papachristou had been excluded from the team "for statements contrary to the values and ideas of the Olympic movement."
"It's the same as violating fair play," Isidoros Kouvelos, the head of Greece's Olympic mission, told the country's Skai TV channel. "We are not here just to get medals, but to promote the Olympic ideals, to show our character."
He added that the Hellenic Olympic Committee did not contact Papachristou either before or after issuing the statement excluding her from the games.
Papachristou is in Athens and had been set to travel to London shortly before the track events start.
After the comments and the ensuing uproar, the Hellenic Olympic Committee banned all Greek athletes from using social media to express any personal opinions not related to the Olympics.
"They can't express personal opinions on other, third subjects, but only about themselves, their athletic condition, if they're on form, or about the games, until the games are over," committee spokesman Tassos Papachristou — who is not related to the athlete — told The Associated Press in London.
The decision to pull the 23-year-old triple jumper from the Olympic team was irreversible, he said.
"She has been informed ... that this is a decision that will not change," he said.
Papachristou's Twitter account ((at)papaxristoutj) contained several retweets and links to sites and YouTube videos promoting the views of Golden Dawn, a formerly marginal extreme right party that won Parliamentary seats in national elections in May and June, polling almost 7 per cent of the vote. She has since erased those links and retweets from her account.
But it was her attempt at a joke Sunday that went viral. Commenting on the widely reported appearance of Nile-virus-carrying mosquitoes in Athens, Papachristou wrote: "With so many Africans in Greece, the West Nile mosquitoes will be getting home food!!!". Her tweet prompted thousands of negative comments that snowballed Wednesday.
International Olympic Committee communications director Mark Adams said the issue was under the jurisdiction of the national Olympic Committee.
"They clearly felt that the Olympic Charter had been contravened, and that is their decision," he said at a briefing in London, adding that rules on athlete behaviour are well established. "The process seems to have been followed correctly."
According to the Olympic Charter, "any form of discrimination with regard to a country or a person on grounds of race, religion, politics, gender or otherwise is incompatible with belonging to the Olympic Movement."
Since anyone can access an unprotected Twitter account, Papachristou's YouTube links and retweets inevitably became known. Several of her retweets were original tweets by Ilias Kasidiaris, the Golden Dawn spokesman and one of the party's 18 Parliament members, who became notorious a few weeks ago for striking a woman Communist MP in the face and throwing water at another female MP during a TV talk show. Papachristou tweeted to Kasidiaris on his name day, last Friday, "Many happy years, be always strong and true!!!" That tweet has now been erased.
Papachristou's initial reaction to the negative comments, on Tuesday, was to tweet: "That's how I am. I laugh. I am not a CD to get stuck!!! And if I make mistakes, I don't press the replay! I press Play and move on!!!"
Her attitude changed completely Wednesday and she posted six apologetic tweets in less than two hours. The last tweet, a long one in English which she has also posted on her Facebook account, expressed "my heartfelt apologies for the unfortunate and tasteless joke I published on my personal Twitter account. I am very sorry and ashamed for the negative responses I triggered, since I never wanted to offend anyone, or to encroach human rights.
Before the publication of the last tweet, Democratic Left, one of the three parties in Greece's coalition government, published a statement assailing the "racist humour" and calling on the Hellenic Olympic Committee to expel Papachristou from the Olympics.
"Let her make any miserable 'jokes' on social media while watching the games on TV. She definitely cannot represent Greece in London," the party said.
The socialists, another coalition government partner, released a much milder statement saying that "issues touching on human dignity should not be treated lightly. ... We hope that Ms. Papachristou has, though belatedly, realized her mistake and her public apology is sincere."
Greece's track and field federation applauded the decision to exclude Papachristou from the Olympics and announced she would face a disciplinary panel.
Papachristou's coach, Georgi Pomaski of Bulgaria, said the punishment was too severe.
"She did something childish," Pomaski said. "I respect the decision, but it is a little harsh for a kid we are trying to educate."
Her sports club, AEK, also said the exclusion from the Olympics for "a mistake" was hypocritical.
"We will fight any attempt to exclude our athlete ... by all legal means," the club said in a statement.
Elena Becatoros and Graham Dunbar in London contributed to this report.