The 51-year-old man, referred to as AWA, was admitted to the Queen Elizabeth the Queen Mother Hospital in Margate, Kent, on September 7, last year.
AWA, who has dementia and is fed by a tube through his stomach, stayed at the hospital until September 26.
But despite daily visits by his family and carer and meetings between his parents and the clinicians, it was only when AWA returned to his residential home that the DNR order was discovered.
It instructed staff not to perform resuscitation in the event of a cardio or respiratory arrest with no provision of review.
It also gave his disability as the sole reason for its imposition.
Merry Varney, from the human rights team at Leigh Day & Co solicitors which are representing AWA, said: “This is definitely one of the most extreme cases we have seen of a DNR order being not only imposed on a patient without consent or consultation, but to use Down’s Syndrome and learning difficulties as a reason to withhold lifesaving treatment is nothing short of blatant prejudice.
“If an individual was physically preventing a doctor from administering life-saving treatment to a disabled relative, it would undoubtedly be a matter for the police, yet we see doctors taking this decision without consent or consultation regularly.
“The absurdity of this is highlighted by the contrast with cases where people wish to end their own lives.
“Any doctor assisting patients with capacity such as Tony Nicklinson or our own client ‘Martin’, who wish to end their own life would face prosecution for a serious criminal offence.”
Mr X, a family member who is acting as the litigation friend for AWA, said: “We were all shocked to find out about what had been put into AWA’s notes without our knowledge.
“One member of the family at least was in the hospital practically every day and could have been consulted about the decision.
“We are bringing this action to highlight the issue and to make sure that something like this cannot happen to another loved son and brother.”
Ms Varney said she was also leading another case aimed at creating a nationwide policy for DNR orders rather than leaving them to individual NHS trusts.