England and Scotland football players will wear poppies on black armbands during their Armistice Day World Cup qualifier at Wembley in defiance of a ban by the sport's world governing body.
Fifa has insisted that that the laws of the game, which prohibit commercial, political or religious messages from players' kit, mean poppies cannot be worn at the November 11 fixture.
But the English Football Association and its Scottish counterpart have issued near identical statements insisting poppies do not represent such a message, and that a 2011 compromise which allowed them to be displayed on armbands does not break Fifa rules.
Their decision comes after Theresa May was warned by Fifa not to interfere in the row, after the Prime Minister described the ban as "utterly outrageous.
The English FA said in a statement: "We fully respect the laws of the game and take our founding role on the International Football Association Board (IFAB) extremely seriously.
"The poppy is an important symbol of remembrance and we do not believe it represents a political, religious or commercial message, nor does it relate to any one historical event.
"In keeping with the position agreed with FIFA back in 2011 and in what we believe is in accordance with law 4, para 4, the FA intend to pay appropriate tribute to those who have made the ultimate sacrifice by having the England team wear black armbands bearing poppies in our fixture on Armistice Day."
Shortly after the FA's statement, its Scottish counterpart issued an almost identical statement of its own to say Scotland players would also wear armbands.
Fifa secretary general Fatma Samoura, who is from Senegal, had earlier ignored widespread calls to make an exception to the rules.
She told the BBC: "You could make many exceptions. Britain is not the only country that has been suffering from the result of war.
"Syria is an example. My own continent has been torn by war for years. Why are we doing exceptions for just one country and not the rest of the world?"
When asked if she had a message for the PM, Ms Samoura said: "No message to her. I would just ask anybody to refrain from political interference."
At Prime Minister's Questions, Mrs May tore into the scandal-hit footballing body as she defended the players' right to wear special kits.
The PM told MPs: "I think the stance that has been taken by Fifa is utterly outrageous.
"Our football players want to recognise and respect those who have given their lives for our safety and security. I think it is absolutely right that they should be able to do so."
She said it was a matter for the English and Scottish Football Associations, but there was a "clear message" from the House of Commons that "we want our players to be able to wear those poppies".
And in a direct message to world football's governing body, which has been plagued by corruption allegations, she said: "Before they start telling us what to do, they jolly well ought to sort their own house out."
The England and Scottish FAs' moves come after former culture, media, and sport secretary John Whittingdale insisted the two home nations should defy Fifa and risk a potential points deduction.
Condemning Fifa's stance, he told BBC Radio 5 Live: "For them to try and brand the poppy as a political symbol shows a total misunderstanding, and I think there are a number of reasons why we are already profoundly unhappy with Fifa's behaviour and conduct and this adds to that list."
Asked if the teams should risk a points loss, Mr Whittingdale replied: "Yes."
A petition set up by former RAF pilot and prisoner of war John Nichol on change.org, urging Fifa to change its mind, has garnered more than 280,000 signatures.
He wrote: "The poppy is not a political statement at all. It could not be further from a political statement.
"It is a statement of remembrance and an acknowledgement of sacrifice from the First World War right through to the sacrifices of our young men and women today."
Mr Nichol added: "No-one should ever be banned from wearing a poppy and it brings shame on Fifa that they continue to propagate this misunderstanding of our heritage."