There are "serious issues" over the idea of lowering the standard of proof needed to convict terror suspects, says Attorney General Lord Goldsmith.
Goldsmith said liberties and security had to be balanced
The Home Secretary this month said the possibility needed to be debated.
In a speech on Wednesday, Lord Goldsmith said any plans would have to be lawful and meet Britain's international treaty obligations.
There were "no firm proposals", he stressed, but current anti-terrorism powers had to be renewed in 2006.
Tony Blair this week suggested the standard of proof could change in trials of organised criminals too.
Speaking at the Institute for International Affairs in London, Lord Goldsmith said the UK Government was committed to taking the steps necessary to combat terrorism while upholding the rule of law and fundamental human rights.
The attorney-general said Mr Blunkett would next month publish an options paper on possible new terror powers.
Lowering the standard of proof in criminal trials from "beyond all reasonable doubt" to "on the balance of probabilities" is one of the ideas being considered.
Asked about the standard of proof, he said: "I can assure you that there are no firm proposals to adjust the burden of proof.
"I understand the home secretary to have been raising issues which need to be subject to an open debate, particularly in the context of the continuing concerns about security and especially the sunset clause which means the part four powers in the Anti-Terrorism Act will come to an end in 2006.
"Anything proposed would need to be lawful and would need to be in accordance with international obligations."
Lord Goldsmith, the government's chief law officer, said he would be consulted over any plans.
The prime minister this week suggested organised crime could be another area where the standard of proof was changed.
"To require everything beyond reasonable doubt in these cases is very difficult," said Mr Blair.
"I think people would accept that within certain categories of case, provided
it's big enough, you don't take the normal burden."
In his speech, the attorney-general said the 2001 US terror attacks had changed the landscape of terrorism.
There needed to be tough action against terrorists who had no respect for the rule of law - but the values of democratic societies had to be protected, he argued.
He defended the detention of 14 international terrorism suspects who are being held indefinitely in English jails, saying they had no right to be in Britain and could return voluntarily to their homeland.
Such measures had so far been upheld by the courts, he said.
Two suspects have left Britain voluntarily but under human rights laws the rest cannot be removed if they might face death or torture.
A report into the use of those laws on Wednesday said greater efforts should be made to deport them.
In his independent review of the measures, Lord Carlile of Berriew said there was a "strong case" for deportation if there was a "very low risk" that the men would be ill-treated.
He said the evidence showed the home secretary had correctly used his powers to detained the men, but alternatives to custody, such as tagging, curfews and surveillance needed "continuing scrutiny".