DAVID Cameron has said the length of time since the man convicted of the Lockerbie bombing was freed is a reminder of a wrong decision taken by the Scottish Government and an insult to the victims' families.

Abdelbaset Ali Mohmed al Megrahi is set to mark on Wednesday the thousandth day since his release. He was allowed to return to Libya by Scottish Justice Secretary Kenny MacAskill on compassionate grounds because he was suffering from prostate cancer.

It is believed his survival is linked in part to treatment with a cancer drug, Abiraterone, not routinely available on the NHS in Scotland.

Prior to his release, doctors predicted Megrahi had three months to live but as the landmark date approaches, the Prime Minister told The Herald: "One thousand days on, this is yet another reminder that Alex Salmond's Government's decision to free the biggest mass murderer in British history was wrong and an insult to the families of the 270 people who were murdered."

Labour and the Scottish Conservatives also questioned Mr MacAskill's decision while Liberal Democrat leader Willie Rennie called for an investigation into claims crucial information was withheld from Megrahi's trial.

Labour justice spokesman Lewis Macdonald said: "Every anniversary and milestone reached by the man responsible for Scotland's worst-ever act of terrorism must be a grim reminder for the families of the Lockerbie victims. Given that Megrahi was given roughly 100 days to live at the time Kenny MacAskill chose to release him, it is astonishing that he is now set to reach 1000 days and calls into question the reasons for the Justice Secretary's decision."

Scottish Conservative chief whip John Lamont said it was "an embarrassing milestone" for the Scottish Government, and that with every week that goes by, the release decision "looks more and more outrageous".

Mr Rennie said: "However evil Megrahi is, however badly the SNP handled his release and however long he has survived, rather than obsessing about whether a dying man is dead yet we should be investigating whether crucial information was withheld from the trial."

A spokesman for Mr MacAskill said extensive scrutiny had vindicated the decision.

"Whether people support or oppose the decision, it was made following the due process of Scots law and we stand by it: al Megrahi is an extremely sick man dying of terminal prostate cancer."

The spokesman said "substantial opinion" including that of Nelson Mandela and Dr Jim Swire, whose daughter died in the airline bombing, supported the decision.

He said the Government respects the views of those who oppose it, but regardless of those views "they can have complete confidence that it was taken on the basis of Scots law".

Janssen-Cilag, the company which manufactures Abiraterone, has re-submitted it to the Scottish Medicines Consortium (SMC) after having it rejected earlier this year because it was not value for money.

A 30-day supply costs £2930 for 120 tablets. The decision means it is now up to consultants to apply for the drug on a case-by-case basis through the Individual Patient Treatment Request scheme if they believe a particular patient would benefit from it.

The SMC told the maker it could re-submit a bid with "new information, such as new evidence or an adjustment to the price".

Meanwhile, the equivalent medical body for the NHS in England and Wales, Nice, is set to issue its own guidance on the use of the drug next month.

Abiraterone is seen as a last chance for prostate cancer sufferers who have exhausted all other treatments.

It is difficult to say how big a role it has had in extending Megrahi's life as trials showed it increased life expectancy by just three months on average.