The Guantanamo Bay detainee David Hicks has the legal right to become a British citizen.
The British Court of Appeal has refused to consider further appeals from the British Home Office.
It means the Home Office must resume the process of declaring Mr Hicks a British citizen after losing two appeals.
His lawyer, Major Michael Mori, says it is now up to the United States to allow the final step for his client to become a British citizen.
"I mean, it's really up to the Department of Defence of the United States. Are they going to interfere anymore or not? Or will they step aside and let the British government representatives get down there? Just five minutes with David, and they can administer the oath."