A private investigation, conducted by a former senior US prosecutor and ex-Metropolitan Police officer, concluded that Mohammed Abu Talb, an Egyptian militant, should have faced trial for the atrocity instead of Megrahi.
Contents of the 2002 report were unearthed by investigative website Exaro.
Jim Swire, whose daughter Flora died when Pan Am Flight 103 exploded over Lockerbie on December 21, 1988, and who has campaigned for the case to be re-investigated by police, said the findings reflected his view that Talb played a "crucial role" in the atrocity.
Jessica de Grazia, New York's former chief assistant district attorney, and Philip Corbett, a former chief security advisor to the Bank of England who served as a high ranking anti-terrorism officer at Scotland Yard, compiled their report for a London-based firm of private investigators, Forensic Investigative Associates, who were commissioned by Megrahi's lawyers.
According to Exaro, they concluded the original police probe was "directed off course as a result of government interference".
The private investigation, codenamed Operation Bird found evidence linking Mohammed Abu Talb with the bombing.
Initially a suspect, he gave evidence for the prosecution at Megrahi's trial and was granted immunity from prosecution. He denied involvement.
He later served a lengthy prison sentence Sweden after being convicted of carrying out a series of terrorist bombings in 1985 in Copenhagen, Denmark and Amsterdam, Holland.
Ms De Grazia and Mr Corbett's report was to have been used in a second appeal by Megrahi, eventually abandoned prior to his controversial release on compassionate grounds by the Scottish Government in 2009. He died of cancer in his native Libya last year.
This Saturday marks the 25th anniversary of the Lockerbie bombing, which killed all 259 flight passengers and crew and 11 people on the ground.
Mr Swire said: "Talb is a life-long, proven terrorist. He has completed 20 years in prison for bombings in Scandinavia, and is now out of prison and living in Uppsala in Sweden.
A Scottish Government spokeswoman said: "Any issues raised in relation to the conviction itself must be a matter for a court of law - Mr al Megrahi was convicted in a court of law, his conviction was upheld on appeal, and that is the only appropriate place for his guilt or innocence to be determined.