David Hicks, who has been charged with terrorism-related offences after he was captured in Afghanistan in 2002, won the right this month to a British passport, which he hoped would force the UK to aid his release from the US camp in Cuba.
The 30-year-old Muslim convert from Adelaide, who is accused by US authorities of attending terrorist training camps in Afghanistan and Pakistan and fighting alongside the Taliban, has a British-born mother.
A spokesman for Britain’s Home Office said: "The ruling by the courts was a disappointment. The Home Office was granted permission to make an appeal and we have done so."
He added that a date for the appeal hearing had not been set.
A judge in London ruled two weeks ago that Home Secretary
Charles Clarke had "no power in law" to deprive Hicks of his citizenship.
Justice Collins had given the Home Secretary permission to appeal against his judgment, but refused to suspend his decision pending the appeal.
Charges denied
Hicks denies charges including conspiracy to commit war crimes and aiding and abetting the enemy.
He said the Australian government had refused to plead for his release and prevent his trial by a US military commission.
By contrast, the British government considers the US military commissions do not guarantee a fair trial in accordance with international standards and has secured the release from Guantanamo Bay of nine British citizens.
The Home Secretary accepted Hicks was entitled to British citizenship but it was argued that registration could be refused, or citizenship withdrawn, because of his alleged involvement with al-Qaeda and terrorist activities against the UK.
The Home Secretary has powers under the 2002 Nationality,
Immigration and Asylum Act and the 1981 British Nationality Act to deprive a person citizenship if he has shown disloyalty or disaffection towards the Queen or associated with an enemy.
But the judge ruled such events could only be taken into account if they occurred after the person had acquired citizenship.
Not confident
Hicks isn't confident his British citizenship will secure his release from detention at Guantanamo Bay, his father Terry Hicks says.
Terry Hicks, spoke to his son by telephone on Christmas Eve.
"He wasn't 100 per cent, he was pretty down," Terry Hicks said.
"I think he's more or less playing it by ear, the mode he's in is 'whatever happens, happens'.
"He's aware that he's a British citizen, for a little while at least, but he's not overly confident of getting released because of it."
Mr Hicks said the British government's appeal was expected, but would delay his son's bid for freedom.
"The High Court was water-tight, this is just (a) delaying tactic from their end," Mr Hicks said.
He said his son had been transferred to a section of the US military detention facility at Guantanamo Bay known as Camp Delta.