England and Scotland players wore black armbands bearing a red poppy during Friday's World Cup qualifying match at Wembley despite failing to get clearance from Fifa.
World football's governing body turned down a request by both teams to wear the symbol to mark Armistice Day.
Fifa said it had not "banned" the move but "reiterated" rules on displaying "political" statements on shirts.
England and Scotland could now face a points deduction, a fine, or both.
England won the Group F match 3-0 thanks to goals from Daniel Sturridge, Adam Lallana and Gary Cahill.
Northern Ireland's players wore plain black armbands during their 4-0 World Cup qualifying Group C victory against Azerbaijan in Belfast on Friday.
Wales will wear plain black armbands when they face Serbia on Saturday.
The Football Association of Wales says it does not want to risk a financial penalty or points deduction by going against Fifa's rules.
What exactly are the rules and who makes them?
The poppy is a symbol of remembrance for those who have died in conflict and is traditionally worn on and in the days before and after 11 November, which is also known as Armistice Day.
According to the rule-making International Football Association Board, which includes members of the four British FAs, players cannot wear "political, religious or personal slogans, statements or images".
Fifa secretary general Fatma Samoura said last week: "We have to apply uniformly and across the 211 member associations the laws of the game.
"Britain is not the only country that has been suffering from the result of war."
What happens now?
Fifa's match commissioner at Wembley will decide whether to mention the armbands in their official report.
If they do, the case would then go to Fifa's disciplinary committee.
The English Football Association has already said it will contest any fine and believes its "legal position is right and our moral position is right".
But former England right-back Danny Mills believes the FA "has picked the wrong fight" and is "likely to get a fine".
The former Leeds player told BBC Breakfast: "Surely all of the money that has been spent on arguments, lawyers and the fine it may get from Fifa would have been much better being donated to the Royal British Legion.
"It would have done far more good than this needless argument."
The Scottish Football Association believes Fifa is "misinterpreting the rules" and claims the poppy "is not a political statement".