The High Court yesterday upheld the Home Secretary Michael Howard's refusal to grant Jaya Ram Khadka "exceptional leave" to remain in Britain on compassionate grounds.
Mr Khadka and his adopted father Richard Morley are now considering setting up home in Nepal.
Mr Justice Laws said in his ruling that many may regard the Home Secretary's decision to ignore an Immigration Appeal Tribunal recommendation to let Mr Khadka stay as "harsh", but after examining the case it was "perfectly understandable".
Mr Khadka, 20, was brought to Britain by businessman Richard Morley in 1990, honouring a pact he made with the boy's father, Basu Khadka - a policeman in a remote area of Nepal who saved Morley's life when the millionaire collapsed with a punctured lung while on a mountaineering expedition in 1984.
He refused any financial reward, but instead obtained a pledge from Mr Morley to take care of his son if he ever became orphaned.
When the policeman died six-years later, Mr Morley brought his 14-year- old son over to Britain and made him heir to his estate at Clearwell Castle in Gloucestershire.
Mr Justice Laws said that he hoped future explanations by Mr Howard of reasons to deport "may be clearer", but he said that he was "unable to conclude that the Secretary of State's decision in this case was beyond the range of responses open to a reasonable decision-maker."
"Many may regard the result he arrived at as harsh," he added.
Mr Morley has made several attempts to extend his son's stay in Britain since 1990. He took his case to the Immigration Appeals Tribunal which made a recommendation that he be allowed to stay on compassionate grounds, but in March this year the Home Secretary signed a deportation order.
Outside the court yesterday, Mr Morley said he would not be separated from his adopted son.
"Wherever Jay goes I will go and the rest of my family will follow. My family will no doubt be split for a period, but we'll have to endure that. Eventually we will be reunited in a different country.
"There is no point in appealing if the law is that the Home Secretary can do what he wants, then that is the law and I cannot change the law".
Mr Khadka said that he felt heartbroken: "I think it is terrible not just for me but for my family.
"The fact my family has stood by me like this for the past six years and will be standing by me for the rest of my life, surely that will speak volumes of compassion."Reuse content