It is the worst of outcomes: shoddy, underhand, secretive, unexplained. That the Lockerbie atrocity — with its savage death toll and the 21 years of suffering that followed — should conclude with this hole-and-corner deal is frankly shameful. A convicted terrorist is allowed to return to his country, while the relatives and friends of those who died are deprived of their last slender chance of learning the truth about what happened.

There are already too many unanswered questions about Lockerbie. Now, we are burdened with what looks suspiciously like a cover-up. By withdrawing his appeal, Abdul Baset Ali al-Megrahi has left the whole issue of his involvement in the attack unexplained — a black hole of charge and counter-charge that will never be tested in court.

Over four years, his defence team has been leaking hints about the strength of its case, and what it saw as the inadequacies of prosecution evidence. To turn its back now, and abandon the whole process, suggests the Libyan is more interested in his short-term security than clearing his name.

The Scottish government’s position is morally even weaker. It has argued all along that its motivation has been a humanitarian one — that allowing al-Megrahi home to die is the act of a compassionate administration, and that it would be insensitive to ignore the pleas of his family, and allow him to die in prison.

That position could only be sustained if, at the same time, it was able to assure the relatives and friends of the victims that the appeal would carry on in his absence, and that the strength of the case against him would be tested in court.

That fiction has now evaporated. The appeal is being dropped, and the Justice Secretary is looking ham-fisted, duped possibly by the Libyans, or worse, complicit with them in a deal where they get their man back, and he is saved the embarrassment of an appeal that might have gone in the Libyan’s favour. For this is the simple outcome: most people who have followed the Lockerbie affair will be convinced that a deal has been struck to avoid the embarrassment of a final appeal. Either it would have upheld al-Megrahi’s conviction, or exposed the fragility of the prosecution case, exposing the Scottish judicial system to ridicule. In both cases political face would be lost.

This, after all, was the highest profile case Scottish judges have ever heard. Its status — in a Scottish court, but on foreign soil — was unprecedented. It rested on one foundation — the unimpeachable reliability of the Scottish legal system.

That reliability was challenged almost from the moment al-Megrahi was convicted and sentenced. Campaigners on his behalf began to unpick central aspects of the prosecution case and — though the wilder conspiracy theories were dismissed — there was still doubt about the scientific trail on which the case rested.

All that, however, was to be addressed by the Scottish Criminal Cases Review Commission, which not only allowed a fresh appeal, but indicated that the defence would be permitted wider grounds for advancing new evidence than normal. It was, in short, the vindication of the Scottish judicial system, and most observers expected it to dispose of the principal doubts in the case. Now that opportunity is gone.

The coincidence of the Justice Minister’s decision to consider releasing al-Megrahi on compassionate grounds, together with the almost immediate announcement that he was dropping his appeal, is too close for comfort. It will convince sceptics that the prosecution case was falling apart, and had to be abandoned.

It will play into the hands of those who believe this is a stitch-up. And it will expose the Scottish government to charges of duplicity or naivete. For the relatives of the victims it is a betrayal. For anyone who believes in the reliability of the legal system it is a let-down. And for those attempting to shore up trust in government it is simply a farce.

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