A judge released one suspect, a 37-year-old Algerian referred to only as "A", on bail late on Thursday and said he intended to free eight others soon.
They include Abu Qatada, a Syrian cleric who the government says was a spiritual mentor to Muhammad Atta, leader of the hijackers who staged the attacks of 11 September 2001.
The suspects will be electronically tagged and closely monitored in their homes, but their imminent release on bail has nevertheless caused concern.
Britain's most senior police officer, Sir Ian Blair, said their release would be "a grave threat to national security".
The men, mostly North Africans and all Muslim, have been held in prison for up to three years without being charged or tried. Senior judges deemed their detention illegal late last year, prompting fresh appeals for their release.
They were arrested under powerful security legislation that Britain passed in the wake of 11 September. That legislation is to expire on Sunday and politicians are locked in a bitter battle over new laws to replace it.
Prime Minister Tony Blair is trying to ram a bill through parliament before the old laws expire, but opponents from both left and right - many from within Blair's Labour Party - say it is badly drafted or draconian.
They also say they have not had time to debate it properly and some are pushing for "a sunset clause" in the bill which would mean it would lapse later this year, allowing legislators to replace it with something better.
Blair has accused the opposition Conservative Party of being irresponsible by insisting on such changes.
"It is time to get serious," Blair told a news conference on Thursday. "We are talking about an issue where the advice is clear. We need these powers."
Blair has managed to push the bill through the elected House of Commons after making major concessions, notably allowing judges to review politicians' decisions to impose "control orders" on suspects.
"It is time to get serious"
Tony Blair, British prime minister
But the unelected House of Lords, where Blair does not have a majority, has rejected large parts of the bill, repeatedly sending it back to the Commons.
In an extraordinary standoff, politicians have sent the bill back and forth between the two houses.
The Commons debated the bill into the early hours on Friday and the Lords, many of them elderly, were due to return to it at 5am (0500 GMT).
By convention, the Lords are expected to back down and bow to the will of the elected chamber, but they appear to be in no mood to uphold tradition.