The UK government has urged Libya not to celebrate the first anniversary of the release of the Lockerbie bomber.
Scottish ministers released Abdelbasset Ali al-Megrahi, who has cancer, citing medical advice that three months was a "reasonable" life expectancy estimate.
He got a hero's welcome in Tripoli and the Foreign Office said similar scenes would be deeply insensitive to families of the 270 people killed in 1988.
The Scottish government said the decision was taken in good faith.
Meanwhile, two US senators are preparing to release what they say is evidence of commercial pressure behind the release of Megrahi.
Senators Robert Menendez (D-NJ) and Frank Lautenberg (D-NJ) said they would publish the details at a news conference which will also be attended by relatives of those who died in the 1988 bombing.
The decision by Scottish Justice Secretary Kenny MacAskill to release Megrahi, who was jailed in 2001, prompted widespread criticism from political opponents and many of the US relatives of those who died.
US president Barack Obama has described the decision as "a mistake", while UK Prime Minister David Cameron has also questioned the move.
In a statement ahead of the anniversary of Megrahi's release, a Foreign Office spokeswoman said: "The government is clear that Megrahi's release was a mistake.
"Both the current Prime Minister and the Foreign Secretary made this clear at the time. Particularly on this anniversary we understand the continuing anguish that Megrahi's release has caused his victims, both in the UK and the US.
"He was convicted for the worst act of terrorism in British history."
In letters to Mr Cameron and Scottish First Minister Alex Salmond, the four US senators setting up an inquiry into issues around the release have reiterated what they call the "persisting uncertainty about medical, legal and diplomatic issues related to" the release.
First Minister Alex Salmond: "We have been told by Libya that no party has been planned"
The BBC's Matthew Price said they want "more information on the medical opinions that led to the conclusion that Megrahi had just three months to live and details on communications between BP and the British government".
It follows an earlier decision by the Scottish government not to send officials to a hearing in Washington.
Mr MacAskill has said that he is prepared to meet the group of US senators who are investigating whether BP influenced the decision to free Megrahi to help it clinch oil deals with Libya.
He said: "If US senators come across and they seek a meeting with me, I'll be more than happy to try and provide it.
"I did meet with congressmen back in February, indeed the door is always open."
Mr MacAskill said he made his decision after receiving medical advice that a three-month prognosis for the bomber to live was a "reasonable estimate".
However, speaking on the eve of the one-year anniversary of the Libyan release, Scottish Labour leader Iain Gray said: "How much compassion did his government show to the American relatives of the 270 people killed at Lockerbie."
And Murdo Fraser, the Scottish Conservative deputy leader, has used the anniversary to call for full disclosure of the facts surrounding Megrahi's release.
Pan Am Flight 103 from London to New York exploded 31,000ft over Lockerbie 38 minutes after take-off.
The 259 people on board the Boeing 747 were killed, along with 11 people on the ground.
BBC News, Tripoli
This time last year, Libyans were cheering on the tarmac of a Tripoli airport when Abdelbasset Ali al-Megrahi returned. But a year on a repeat of such euphoric scenes seems unlikely and he has dropped into obscurity.
His condition, as far as we are aware, has not dramatically changed. His brother-in-law told the BBC that Megrahi spends most of his time at home still undergoing treatment for his illness.
Libyan observers believe that the recent controversy over Megrahi's release has been reignited by US senators with upcoming midterm elections in mind.
And the question over how long Megrahi has to live is one that people generally find offensive. Many Libyans view with bewilderment, and at times anger, what seems to them like a transatlantic countdown to his death.
He continued: "What's quite clear is the people of Scotland think the decision should be made here in Scotland, by the justice secretary, and they do believe it was made in good faith without any intervention, or indeed any consideration, of political, diplomatic or economic considerations."
He has always maintained the decision to release Megrahi followed due process and was in keeping with the ideals of the Scottish justice system.
Mr Salmond said the release had been based on the information available to the Scottish government and that it was hugely difficult to be exact in terms of the prognosis of someone with terminal cancer.
He told BBC Radio Scotland's Good Morning Scotland programme: "Everybody knows it's always difficult because they will have friends and relatives who've been through the same experience of trying to exactly define the life prognosis of somebody with terminal cancer."
He added: "The prognosis comes from Dr Andrew Fraser, who's the head of medicine at the Scottish Prison Service.
"He compiled the information, as he said in his report, from the range of evidence, the medical notes, the case notes - that includes a range of doctors, it includes the palliative care team, it includes the primary care physician and obviously if he needs to have the external reference, he described as the external advisor, he turned to one of the most prominent oncologists that Scotland has ever produced."
Since Scottish devolution, 27 prisoners have been freed on compassionate grounds. Of those, only two including Megrahi lived for longer than a year after their release.
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