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 mark Armistice Day by wearing the emblem.
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.
Northern Ireland’s players wore plain black armbands for their qualifier against Azerbaijan in Belfast, while fans held up a poppy banner in the West Stand.
Wales, who also had a request to wear the poppy “turned down” by Fifa, will wear plain black armbands when they face Serbia on Saturday.
The Football Association of Wales said it could not risk a financial penalty or point deduction by defying the rules for the World Cup qualifier.
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 International Football Association Board (Ifab), which includes members of the four British FAs, players cannot wear “political, religious or personal slogans, statements or images”.
If Ifab decides there has been a breach of these rules, Fifa’s disciplinary committee decides the punishment.
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.”
The Scottish Football Association believes Fifa is “misinterpreting the rules” and claims the poppy “is not a political statement”.
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”.
How has football reacted?
England interim-manager Gareth Southgate said he wanted his side to wear the poppy during the game with Scotland.
But former Three Lions right-back Danny Mills believes the English 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.”
What happens next?
The first stage in any disciplinary process that might follow Friday’s game would be for Fifa’s match commissioner at Wembley to make mention of the armbands in the official report.
The case would then go to the disciplinary committee, which would announce its decision within a few weeks.
The British FAs would have an opportunity to challenge any fine via Fifa’s appeals process and have a further chance to appeal at the Court of Arbitration for Sport, a course of action that would probably cost more than the fine.