Statements about a break-in at Heathrow airport just hours before the December 1988 bombing were kept by Dumfries and Galloway Police until 1999.
A letter from Pat Shearer, the current chief constable of the force, has finally admitted the police delay.
Mr Shearer also reveals the Crown Office knew about the break-in before the trial, but failed to tell the defence team.
Since 1991, police and prosecutors team had maintained the bomb which exploded on board Pan Am 103 over Lockerbie – killing 270 people – was placed on a flight from Malta, rather than at the London airport. This underpinned the case against Abdelbaset Ali Mohmed al Megrahi, the Libyan convicted of the bombing in 2001.
Dr Jim Swire, whose daughter Flora died in the atrocity, believes the revelations would have undermined the trial. If the bomb was taken on at Heathrow, and had this information been shared, Dr Swire believes the investigation would have pointed elsewhere.
The Herald revealed yesterday that Frank Mulholland, the Lord Advocate, and Robert Mueller, the head of the FBI, travelled to Libya last week to smooth the way for police to pursue inquiries in the country. However, Dr Swire questioned the validity of the trip, based on what he believes is an outdated view Libya was responsible for putting the bomb on the plane in Malta.
Another theory is that Syrian explosive devices could have been put aboard at Heathrow.
Dr Swire said: "The evidence of barometric devices from Syria was rejected because they would have to have been put on the flight at Heathrow. We did not know then about the break-in.
"If it had been known about before the trial there would have been no prospect of getting a conviction. It has relevance to the foundations on which Mr Mulholland bases his approach to the interim Government in Libya, since the intention of his investigation still seems to be to link the conviction of Megrahi to the survivors of the Gaddafi regime. What about looking elsewhere?"
Mr Shearer's letter states no "suppression of evidence took place" but points out Dumfries and Galloway investigated the break-in in January 1989 and did not pass statements – including those from Ray Manly, then head of security at Heathrow's operators BAA – to the Crown until 1999.
The letter also reveals the Crown did not share this information until after Mr Manly himself approached the defence team in 2001.
The Crown has pointed out the break-in was raised at Megrahi's first appeal in 2002 but was considered insufficient to overturn the conviction.
A Crown Office spokesman said: "The Appeal Court was satisfied that, having heard direct evidence about the break-in at Heathrow, the verdict of the trial court was not a miscarriage of justice.
"Even if the evidence about Heathrow had been heard by the trial court, it would not have reached a different verdict.
"The Crown was in the process of robustly defending the investigation and conviction when Mr Megrahi chose to abandon his second appeal."