"I speak with the authority of the government," Lord Falconer told the BBC ahead of a speech in Sydney, saying he had Mr Blair's backing for his comments.
Lord Falconer said the camp violated a fundamental legal principle that courts be able to review the conduct of the executive.
"Otherwise the conduct of the executive is not defined and restrained by law," Lord Falconer said in a speech prepared for delivery at the Supreme Court of New South Wales in Sydney.
A transcript of the speech was released in advance by his office.
"It is because of that principle, that the USA, deliberately seeking to put the detainees beyond the reach of the law in Guantanamo Bay, is so shocking an affront to the principles of democracy," he said.
In June, Lord Falconer denounced the camp as a "recruiting agent" for terrorism, and described its existence as "intolerable and wrong."
In May, Attorney General Lord Goldsmith called for the closure of the camp.
"It is time, in my view, that it should close ... The historic tradition of the United States as a beacon of freedom, of liberty and of justice deserves the removal of this symbol," Lord Goldsmith said.
Mr Blair, a stalwart ally of US President George W Bush, has been more cautious in criticising the camp, calling it an "anomaly" that eventually must end and calling for understanding of the threat the US faces and the fact that many of those imprisoned were picked up on battlefields in Afghanistan.
The Guantanamo detention centre in eastern Cuba holds some 460 detainees, including 14 top alleged al-Qaeda figures recently transferred from CIA custody. Among them is Khalid Sheikh Mohammed, the alleged mastermind of the September 11 attacks.
Only 10 Guantanamo detainees have been charged with crimes so far, including Australian David Hicks.
The camp came under worldwide condemnation shortly after it opened more than four years ago.
The criticism intensified after reports of prisoner abuse, heavy-handed interrogations, hunger strikes and suicides.
Britain successfully sought the release of nine British citizens who had been held at the camp. All have returned to Britain, and none was charged with a crime.
Lord Falconer said the government had raised its concerns privately with the Bush administration.
"The USA is a close and staunch ally of the United Kingdom. We have over time raised the issue in private, we have also sought to protect the interest of our own nationals that were in Guantanamo Bay, all of whom are now back," Lord Falconer told the BBC.
"As time goes on, private words have to get replaced to some extent by public views," he said.