Torture by Proxy: International law applicable to 'Extraordinary Renditions', UK All Party Group on Extraordinary Rendition, December 2005 [briefing paper concluding that the British government could be held liable as an accomplice to American wrongdoing under international law if it helped flights transporting detainees]. Excerpt from the Foreward by Andrew Tyrie MP:

This paper shows that there is a real and clear legal imperative to find out what is going on, and to ensure that no state engages in Extraordinary Rendition. This applies to the UK as much as it does to the US — as the authors state plainly: 'seemingly innocuous acts (e.g. allowing refuelling at airports of aircraft of another State) can become wrongful under international law if those acts facilitate Extraordinary Rendition.' Even if the legal arguments were equivocal, the moral case is unassailable: there is simply no justification whatsoever for the UK or the US engaging in torture, whether by direct or indirect means.

Nor is it likely that torturing people, if this is what is going on, can assist in combating terrorism. Information obtained in such a way is said to be highly unreliable. Worse, the knowledge that such practices were being conducted could act as a recruiting sergeant for terrorism. The case for the values of our democratic societies, which we would like to see spread around the world, will be undermined if we do not respect those values.

Read the full text of the report [PDF]. Reported in JURIST's Paper Chase here