Terry Hicks says Britain's decision is based on the flimsy technicality that when his son was captured among Taliban forces in Afghanistan in 2001, he was solely an Australian citizen.
David Hicks, who has been held at the US military prison at Guantanamo Bay, in Cuba, for four-and-a-half years, has since been granted British citizenship because his mother is British.
Britain has successfully lobbied the US to release its citizens detained at Guantanamo Bay by arguing the US military commissions established to try the terrorist suspects are illegal and do not meet international standards of law.
But the British Foreign Office said it would not make such representations on Adelaide-born Hicks's behalf because he was an Australian citizen when captured and handed to US forces, Terry Hicks said.
"It's surprising, I thought we may get a better result," Mr Hicks said.
"They are saying David was arrested as an Australian so they don't need to do anything about it.
"But our advice is that, under Commonwealth laws, it doesn't matter, he's still part of the Commonwealth."
Government backs decision
The Australian government has backed Britain's decision, with Foreign Minister Alexander Downer saying he had briefings with the British government during his visit to London.
Mr Downer said it was entirely a decision for Britain.
"It's really nothing to do with us," Mr Downer told ABC radio from London.
The minister's comments angered Mr Hicks, who implored the Australian government to follow other governments and press for the release of their citizens from Guantanamo Bay.
"I'm getting used to Mr Downer's cynical remarks, he's almost gloating," Mr Hicks said.
"I find it strange that he is in London when the British announce their decision, he may have gone over to dig a hole for David.
"They (the Howard government) have demonised David by saying he's guilty, but the only way for them to prove it is through the military commission system which is set up to find people guilty."
Hicks was captured among Taliban forces in Afghanistan in December 2001 and reportedly sold to US forces by the Northern Alliance. He has been detained at Guantanamo Bay since January
2002.
Hicks pleaded not guilty to charges of conspiracy, attempted murder and aiding the enemy, at a military commission hearing in August 2004. He is yet to face trial.