A group of 10 Nigerian athletes have been declared ineligible for the Olympics by the Athletics Integrity Unit (AIU) because they were not tested rigorously enough in the run-up to Tokyo 2020.
A total of 20 athletes from the seven nations deemed most at-risk when it comes to doping have been ruled out of the Games.
The figure of 10 Nigerians represents more than 40 per cent of the track and field team the country had entered for the Olympics.
Three athletes from Ukraine, one Moroccan, one from Ethiopia and three from Belarus have also been struck off the entry lists.
Two Kenyan athletes were deemed non-eligible, but Athletics Kenya replaced them before submitting its final entries to World Athletics.
Bahrain is the final Category A country, but all 13 of its athletes have been cleared to compete.
Category A National Federations are deemed by the AIU to be the most susceptible to doping so must meet certain extra requirements to be able to select an athlete for the Olympic Games or World Championships.
In this instance, the rejected athletes have all been excluded from the Hames on the basis of Rule 15 of the National Federation Anti-Doping Obligations.
Introduced in 2019, the regulations state that in the 10 months before the Olympics, any athlete from a Category A country needs to face at least three no-notice, out-of-competition doping tests, which must be conducted at least three weeks apart.
The 20 athletes deemed ineligible for Tokyo 2020 were not subjected to these testing standards.
Ese Brume, a bronze medallist in the women's long jump at the 2019 World Championships in Doha, and Blessing Okagbare - a long jump silver medallist at the Beijing 2008 Olympics who qualified for the 100 and 200 metres this time around, are not among the Nigerians ruled out.
The country's participation in the women's 4x100m and mixed 4x400m relays in is major jeopardy, with only two athletes now listed as entered for each race by the International Olympic Committee.
Nigeria was added to the Category A list in 2020, along with Morocco, while the other five nations predate them.
Despite the disqualifications, the AIU insists progress has been made by the Category A territories on the whole.
This includes in excess of 1,600 out-of-competition tests being conducted domestically - and on top of AIU tests on athletes in the International Registered Testing Pool - in the seven countries since the start of 2021.
Increased testing resulted in six adverse findings, per the AIU.
"National Federations must play their part in supporting anti-doping efforts," said David Howman, chair of the AIU Board.
"The eligibility rules for athletes from 'Category A' countries are very clear and compliance is essential for cementing the required long-term changes and ensuring a level playing field for clean athletes.
"I must underline that there have been significant improvements in anti-doping efforts in most 'Category A' countries thanks to this rule.
"It is clear that the relevant National Federations in conjunction with their NADOs [National Anti-Doping Organisations] have started to take their testing responsibilities seriously, and I thank them for their efforts, but there remains a long way to go in some circumstances."
Track and field competition at Tokyo 2020 is due to begin on Friday (July 30).