The government says "very few" people are held in police cells for long
Government plans to enable police to hold terror suspects without charge for 42 days have caused "considerable concern" at Europe's human rights body.
The Council of Europe's anti-torture committee said suspects should be taken to prison after 14 days as police cells were inadequate for longer detention.
The controversial law was passed by MPs in June and will be voted on in the Lords in a fortnight.
The government said it was "acutely aware" of its responsibilities.
The Council of Europe's Committee for the Prevention of Torture and Inhuman or Degrading Treatment or Punishment (CPT) visited the high-security Paddington Green police station in west London in December.
It wanted to inspect "the safeguards afforded to persons detained by the police under the Terrorism Act 2000 as well as the conditions of detention of such persons".
Under the Act, terror suspects can be detained up to 28 days - which the government wants to extend to 42 days in "special circumstances".
But the committee said in a report released on Wednesday: "The existing - and possible new - provisions regarding the permissible length of pre-charge detention in cases falling under the terrorism legislation are a matter of considerable concern to the CPT.
Transfer to a prison should in all cases be obligatory if detention of a terrorist suspect beyond 14 days is authorised
"The committee has no intention of entering into the current debate on the arguments for and against the length of pre-charge detention of terrorist suspects in the UK.
"However, as the CPT has emphasised in the past, in the interests of the prevention of ill-treatment, the sooner a criminal suspect passes into the hands of a custodial authority which is functionally and institutionally separate from the police, the better."
Under the UK's code of practice, suspects must be transferred from police station to prison after 14 days unless a detainee specifically asks to remain in the police station, or when transfer to prison would hinder effective investigation of the case.
The CPT report warned: "The information gathered at Paddington Green high security police station indicates that the exceptions have become very much the rule."
It said that allowing requests was a "fundamentally-flawed approach from the standpoint of the prevention of ill-treatment".
It also doubted whether an investigation would be hindered if a suspect was transferred, as police can still question the detainee, even in prison.
"Transfer to a prison should in all cases be obligatory if detention of a terrorist suspect beyond 14 days is authorised (and, preferably, such a transfer should occur at a much earlier stage)," the report said.
The committee criticised the state of Paddington Green, especially as authorities had failed to act on its 2005 report, which said the station was not suitable for prolonged detention.
But a Home Office spokesman said: "Very few terrorism suspects have been held in police custody for more than a few days and all have been with judicial authority.
"Of 11 individuals held in custody for 14 days or more, nine were transferred to the prison authorities at 14 days."
The spokesman added that the government takes the welfare of detainees seriously and that safeguards are in place for suspects detained for longer periods.