"The new British government is clear that Megrahi's release was a mistake," Nigel Sheinwald, the UK ambassador to the US, said on Thursday.
Al-Megrahi - the only man convicted of the 1988 airline bombing that killed 270 people - was released from a Scottish prison last August on compassionate grounds after doctors said he was near death.
But nearly one year on, the alleged former Libyan intelligence agent remains alive, reviving outrage on both sides of the Atlantic over his release.
Sheinwald's comments on Thursday came as Clinton agreed to look into claims by a group of US senators that British oil giant BP had lobbied the British government to release al-Megrahi to protect commercial interests in Libya.
The US Senate Foreign Relations Committee has scheduled a July 29 hearing to investigate the claims.
BP signed an exploration agreement with Libya in May 2007, the same month Britain and Libya signed a memorandum of understanding that led to al-Megrahi's release.
The oil company has denied the allegations, admitting that it did lobby the government over a Libyan prisoner transfer agreement but did not enter discussions on al-Megrahi.
Sheinwald, in an open letter to John Kerry, a US senator, published on Friday, also defended BP from the claims.
Meanwhile, a spokesman for the Scottish government said on Friday that it had no contact from BP in relation to al-Megrahi.
"The issues being raised in the United States at present regarding BP refer to the prisoner transfer agreement (PTA) negotiated by the governments of the UK and Libya, and therefore have nothing to do with the decision on compassionate release which is a totally different process.
"We were always totally opposed to the prisoner transfer agreement negotiated between the UK and Libyan governments.
"The memorandum that led to the PTA was agreed without our knowledge and against our wishes.
"The Justice Secretary rejected the application from Libya under the PTA specifically on the basis that the US government and families of victims in the US had been led to believe that such a prisoner transfer would not be possible for anyone convicted of the Lockerbie atrocity.
"[Al-Megrahi] was sent home to die according to the due process of Scots Law, based on the medical report of the Scottish Prison Service Director of Health and Care, and the recommendations of the Parole Board and Prison Governor - all of which have been published by the Scottish Government."