Mr Hague wrote to Secretary of State Hillary Clinton amid renewed questions in Washington over the Scottish Executive's decision to return Abdelbaset Ali Mohmed al- Megrahi to Libya last year.
A group of Democrat senators is demanding an inquiry into claims the oil giant lobbied for the release to smooth a deal and an influential senate committee is also to examine the case.
The fresh attention on BP – already battling the political fallout of the Gulf of Mexico spill – threatens to overshadow David Cameron's first visit to Washington as Prime Minister tomorrow
Mr Hague spoke at length about the issue with his counterpart, promising to "engage constructively" but reiterating the British Government considered the release "a mistake".
In his letter – copied to foreign relations committee chair Senator John Kerry – he said: "There is no evidence that corroborates in any way the allegations of BP involvement in the Scottish Executive's decision to release Megrahi on compassionate grounds in 2009 nor any
suggestion that the Scottish Executive decided to release Megrahi in order to facilitate oil deals for BP."
He expressed "concern and regret at the continuing anguish that the release has caused the families of Megrahi's victims in the United States, as well as in the UK.
"I would like to make clear that this administration believes that the release of Megrahi was a mistake. I said this myself in October 2009 and the Prime Minister also wrote to the then Prime Minister Gordon Brown to express his opposition to the release," he added.
"It remains our view," he said, but insisted that the Government had to "respect the legal and constitutional independence of the process that the Scottish Executive followed".
An inquiry by the Justice Committee of the Scottish Parliament found the Executive "followed due process under Scottish law", he said, and "extensive documentation" relating to the release and the prisoner transfer agreement had already been put in the public domain.
The Foreign Secretary's letter follows a similar declaration by the British ambassador to Washington, Sir Nigel Sheinwald, as diplomatic efforts to ease any row intensify.
He said he had asked Sir Nigel to stay in "close touch" with Senator Kerry's committee to see "what other assistance it would be appropriate for us to give".
Megrahi, the only man convicted of involvement in the bombing of Pan Am flight 103, which killed 270 people in 1988, was freed on compassionate grounds after being diagnosed with terminal cancer.
There was outrage on both sides of the Atlantic when he was flown home to a hero's welcome in Tripoli, and the case was revived as the first anniversary of his release approaches without news of his death, despite having originally being given only three months to live.
BP acknowledges warning the British Government of a potential "negative impact on UK commercial interests" of slow progress being made concluding a prisoner transfer agreement with Libya. But it insists it had no discussions with London or Edinburgh over Megrahi's release.
The transfer agreement with Libya was signed by the former Labour government in 2007 – the same year that BP sealed a 900 million dollar exploration agreement with the North African state.
BP officials are expected to be asked to give evidence to special hearings of the foreign relations committee later this month.
The Scottish government said it had "no contact from BP in relation to Mr al-Megrahi" and said the issues raised in the US related to the UK-negotiated transfer agreement to which it had always been "totally opposed". They had "nothing to do with the decision on compassionate release".
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