Ms Spall with the letter (Picture: SWNS).
Christine Spall found the pencil-written note while sifting through a box of her late husband's belongings.
The letter dated February 27, 1918 and detailed a message from Airman Henry Walter Spall to his second son Horace Henry Spall, Ms Spall's father-in-law.
Ms Spall said: “All these years I had no idea that such a beautiful and special letter that is so old and full of history was hiding in my home.
"I was out in the garden about to mow the lawn and it started raining.
“So I went inside and decided that I would finally have a sort through this box of old things and I found this in an envelope.
"It was in one of those boxes that are just passed over your family
“I thought it might be something interesting but when I saw what it was, I was just gobsmacked."
The letter was headed 'British Expeditionary Force', the British Army personnel that was sent to the Western Front.
After doing some research, Ms Spall found out that Airman Spall served in the Royal Flying Corps and then the RAF.
Ms Spall, from Ipswich in Suffolk, said she didn't know her father-in-law served in the War and believes the letter didn't mention fighting "in case it was intercepted" by enemy spies.
The letter to Mr Spall began: “My dear sonny, many thanks for your kind and loving letter and to say I was glad to hear all of you at home was quite well.
It goes on to mention Mr Spall misbehaving as his father urges his son to improve at school and to do well for the next school report.
The letter ends:
“Well I don’t know what else to say to you now... God bless you lad, from your loving Dad.”
Ms Spall said she knew Airman Spall was a joiner and carpenter who lived in a small two-bedroom house in Ipswich with his ten children but, according to British War Museum records, enlisted in 1918.
It is unknown what year Airman Spall was born in but it is thought he was part of of the RFC force which supported the Allies during a large German offensive in the spring of 1918.
The RFC provided vital air support as the British took part in a controlled retreat.
The RFC and Royal Naval Air Service later merged into the new RAF and continued to provide vital support for the Army.