Europe's human rights watchdog has expressed "considerable concern" about the prospect of terrorist suspects being held in police custody in the UK for up to 42 days.
The controversial law was passed by MPs in June, triggering the resignation of shadow home secretary David Davis.
The legislation must now go to a House of Lords vote scheduled for October 13.
The report says that, even if it goes through, it should be obligatory to transfer terrorist suspects from police detention to prison if they are being held longer than 14 days.
It follows a visit to London's Paddington Green high security police station last December by the Council of Europe's Committee for the Prevention of Torture and Inhuman or Degrading Treatment or Punishment (CPT).
The aim was 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".
The Act allows police detention of terrorist suspects for up to 28 days, but Prime Minister Gordon Brown narrowly won a Commons vote in June to increase the maximum to 42 days "in specified circumstances".
The CPT report warned: "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.
"The information gathered at Paddington Green high security police station indicates that the exceptions have become very much the rule."
