Professor John Tulloch, who lives in the Vale of Glamorgan, south Wales, once held a British passport and has spent most of his life living and working in Britain.
But now semi-retired, the Indian-born academic, whose parents were British, said he has been told he is no longer entitled to remain in the UK indefinitely.
Border Agency officials have questioned his entitlement to a UK passport.
But Prof Tulloch, who until recently discovered he had a lesser form of British nationality known as “British subject without citizenship”, has described the situation as bizarre.
“My wife has a British passport, my sons both have British passports, my brother – who was born in India – has a full British passport but not me,” he added.
“My family goes back in Britain to (the year) 1200 or something. It’s been traced, so what do you do?”
Prof Tulloch became as Australian citizen in 1983, when he took up a post in Sydney.
A UK Border Agency spokesperson said: “If you are a British subject otherwise than by connection with the Republic of Ireland or a British protected person you will lose that status on acquiring any other nationality or citizenship.”
Prof Tulloch was one of hundreds injured in the 7/7 blasts in 2005 when four suicide bombers killed 52 people.