Senior Tory MP Andrew Tyrie and human rights lawyer Anthony Peto, QC, made the warning in a joint report, before the The Justice and Security Bill goes before MPs tomorrow.
It outlines plans to allow court doors to close in England and Wales and evidence to be heard in secret on grounds of national security.
In a report for the Centre for Policy Studies, they said the bill would make it more difficult to discover the extent of Britain's complicity in cases of extraordinary rendition.
Urging the Government to alter the legislation, the report said "ministers still have the scope to enhance, rather than diminish, Britain's moral standing".
It goes on: "That Britain allowed itself to be dragged into complicity in 'extraordinary rendition' – the kidnap and torture of individuals as a matter of policy – is a disgrace.
"That, nearly a decade later, the extent and limits of Britain's involvement are still unknown is almost as shocking. Far from bolstering that confidence, the Justice and Security Bill would weaken it.
"The effect of the Government's proposals would make it more difficult to establish the truth about Britain's complicity in kidnap and torture.
"The bill would provide a route neither more just nor more secure."
Cabinet minister Ken Clarke insisted the legislation was necessary to allow the security and intelligence agencies to defend themselves in court.
Prestwick Airport has been at the centre of the extraordinary rendition controversy, with claims it was used as a refuelling stop for such journeys.