The Government failed properly to investigate claims that the CIA secretly transported al-Qa'ida suspects through Britain to face possible torture overseas, MPs and peers will say today.
They will also denounce Home Office plans to return alleged terrorists to countries with a history of harming prisoners.
Despite reports that 200 American "extraordinary rendition" flights have passed through Britain, the Foreign Office has insisted there have been no US requests to fly terrorist suspects through UK airspace for eight years. But the Joint Committee on Human Rights will say ministers have "not adequately demonstrated" they had looked into the reports as required under human rights legislation.
The cross-party committee is calling for an investigation into the alleged "extraordinary rendition" flights, including who was on them and where they were going.
"We believe the Government should now take active steps to ascertain more details about the flights which it is now known used UK airports," the committee's report, published today, says. "If evidence of extraordinary renditions come to light from such investigations, the Government should report such evidence promptly to Parliament."
The report adds: "The piecemeal way in which information has so far reached the public domain does not inspire confidence in the Government's willingness to investigate."
In February National Air Traffic Services disclosed three US planes associated with "extraordinary rendition" had used British airspace about 200 times in the past five years. The Government refused to provide further details.
The committee criticised the Government's programme of seeking "no torture" agreements with other countries so terror suspects can be deported.
A key part of the Home Office's counter-terror measures, the so-called Memoranda of Understanding (MOU) are designed to circumvent the European Convention on Human Rights, thus allowing suspects to be returned to countries with poor human rights records.
The committee said it feared MOUs could lead to a "substantial risk" of suspects being tortured.