Nigeria's military had advance warning of an attack on the town where some 270 girls were kidnapped but failed to act, Amnesty International says.
The human rights group says it was told by credible sources that the military had more than four hours' warning of the raid by Boko Haram militants.
Fifty-three of the girls escaped soon after being seized in Chibok on 14 April but more than 200 remain captive.
Nigeria's authorities say they "doubt the veracity" of the Amnesty report.
"If the government was aware [beforehand] there would have been an intervention [against the militants]," Nigerian Information Minister Labaran Maku told BBC World TV.
However, he said the authorities would still investigate the claims. 'Gross dereliction of duty' Amnesty says it was told by several people that the military in Maiduguri, capital of the north-eastern Borno state, was informed of the impending attack on Chibok town soon after 19:00 local time.
It says that a local official was contacted by herdsmen who said that armed men had asked them where the Government Girls' Secondary School was located in the town.
Despite the warning, reinforcements were not sent to help protect the town in the remote area, which was attacked at around midnight, Amnesty says.
One reason, the rights group says, was a "reported fear of engaging with the often better-equipped armed groups".
In its report, Amnesty International said the failure of the Nigerian security forces to stop the raid - despite knowing about it in advance - will "amplify the national and international outcry at this horrific crime".
The organisation's Africa Director Netsanet Belay said it amounted to a "gross dereliction of Nigeria's duty to protect civilians" and called on the leadership to "use all lawful means at their disposal to secure the girls' safe release and ensure nothing like this can happen again".
A father of one of two of the missing schoolgirls told the BBC's John Simpson that he believed there was "politics" behind the kidnappings because there was prior information that the militants would be coming to Chibok.
Boko Haram has admitted capturing the girls, saying they should not have been in school and should get married instead.
In a video released earlier this week, leader Abubakar Shekau threatened to "sell" the students.
It is believed the schoolgirls are being held somewhere, perhaps in scattered groups, in the vast forested areas that stretch from near Chibok into neighbouring Cameroon.
Teams of experts from the US and UK - including military advisers, negotiators and counsellors - have arrived in Nigeria to help locate and rescue the abductees.
US Secretary of State John Kerry said earlier that an inter-agency team will work with the Nigerian authorities to secure the girls' release and stressed: "We are also going to do everything possible to counter the menace of Boko Haram".
Boko Haram, whose name means "Western education is forbidden" in the Hausa language, began its insurgency in Borno state in 2009.
At least 1,200 people are estimated to have died in the violence this year alone.
The Nigerian leadership has been widely criticised for its perceived slow response to the girls' kidnapping.
More protests were held in the British capital, London, and Nigeria's main city, Lagos, on Friday.
Speaking to the BBC's World Have Your Say programme, Mr Maku said it was important to remember that the army was not fighting an "easy war" against Boko Haram, which operates over a huge area in the remote north.
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