Fifa has climbed down from its ban on the England team wearing poppies during a friendly match with Spain on Saturday after pressure from both David Cameron and Prince William.
Football's world governing body backed down on Wednesday afternoon, agreeing that England footballers can wear a poppy on black armbands during the match.
It had repeatedly turned down the FA's request that England be allowed to commemorate Remembrance Day with the poppy, arguing it breached its rules on maintaining political neutrality in the sport.
But in his letter Prime Minister David Cameron said there were "no political connotations whatsoever to wearing a poppy" and urged Blatter to reconsider.
"I know that you are aware of the importance of Remembrance Sunday in the United Kingdom, when we pay tribute those who have made the ultimate sacrifice in times of war," he said.
He wrote:
Dear Sepp
I know that you are aware of the importance of Remembrance Sunday in the United Kingdom, when we pay tribute those who have made the ultimate sacrifice in times of war.
We fully understand, and respect, FIFA's rules on its member nations not adorning their shirts with 'commercial', 'political', or 'religious' symbols or messages. However, wearing a poppy is an almost universal symbol throughout the United Kingdom - people from all backgrounds and walks of life across the country join together in doing this as an act of national remembrance, to commemorate those who gave their lives in the service of their country. I can assure you that there are no political connotations whatsoever to wearing a poppy.
You will have seen the letter to you on this issue from Hugh Robertson, the Minister of Sport and Olympics, yesterday. The mood of the House of Commons on this issue today was clear - and I believe this reflects a similarly unambiguous sentiment across the country.
I do hope that we will be able to find a sensible way through which allows British participants in this weekend's matches to commemorate those who have fallen in conflict.
Prince William also wrote to Fifa, in his role as president of the Football Association, to express his dismay at the decision, St James's Palace said.
About an hour after Cameron's letter was published, Fifa backed down and said the wearing of poppies during the game would be allowed.
Prince William said he was “happy” at the decision.
Earlier the PM had called on Fifa to reverse the "absurd" decision that is stopping the team having the remembrance symbol embroidered on their shirts for this Saturday's match.
He stepped in amid growing fury after it emerged that officials turned down the special request, made by the FA, claiming it would "open the door to similar initiatives" across the world.
Cameron said: "This seems outrageous. The idea that wearing a poppy to remember those who have given their lives for our freedom is a political act is absurd.
"Wearing a poppy is an act of huge respect and national pride. I hope Fifa will reconsider."
The England team are being allowed to wear poppies on their training kit at Wembley on Friday and pay their respects to the war dead with a two-minute silence.
Fifa had issued what it a "final statement" rejecting the FA bid for Saturday's friendly against Spain, claiming it would "jeopardise the neutrality of football".
In a letter to the FA, it said: "We regret to inform you that accepting such initiatives would open the door to similar initiatives from all over the world, jeopardising the neutrality of football.
"Therefore, we confirm herewith that the suggested embroidery on the match shirt cannot be authorised. There are a variety of options where the FA can continue supporting the cause of remembrance. One of them already was approved by Fifa, the period of silence."
Jim Boyce, Britain's Fifa vice-president, dismissed claims that the symbol would cause offence. He said: "Personally, I think there has to be a bit of common sense used when requests like this come in. Armistice Day is a very important day in the FA calendar, as it is with other associations, and I don't think it would offend anybody to have a poppy on the shirts."