PRIME Minister Sir Keir Starmer is staring down the barrel of a major Labour rebellion this evening as back-bench MPs have come out against his plan to block a probe into his handling of the Mandelson scandal.One warned that the cover-up could further erode “the already fragile fabric of our democracy.”They are refusing to back his demand that they oppose a Tory motion urging that statements by the Prime Minister be scrutinised by the Commons privileges committee.