In the letter David Cameron said:
Dear Prime Minister,
The decision by the Scottish Justice Secretary to release Mr al-Megrahi on compassionate grounds has been widely condemned, including by President Obama.
As I said yesterday, I believe that the decision to release Megrahi was wrong. He was convicted of murdering 270 people, and I do not believe he deserved to be released on compassionate grounds, or returned to Libya.
The scenes of him receiving a rapturous welcome at Tripoli airport on his return will have distressed many people. I note that Colonel Gaddafi's son has now publicly thanked not just the Scottish authorities but the British Government for its stance, raising questions about the British Government's role.
You have not commented on the decision since it was announced yesterday. This morning your Foreign Secretary refused several requests to say what he thought of the Scottish Justice Secretary's decision.
The fact that the decision to release was taken by the Scottish Justice Secretary does not preclude you, as the Prime Minister of the United Kingdom, from now expressing your opinion on a subject that is of great public concern, and which affects Britain's international reputation and our relations with our allies.
It is curious that while others have commented, Britain's own Prime Minister has not.
I hope you will now take the opportunity to do so.
We are entitled to know what you and your Ministers have said to the Libyan authorities on this matter, and to the Scottish Justice Secretary.
Above all, I believe that the public are entitled to know what you think of the decision to release Megrahi, and whether you consider it was right or wrong.
I hope you will now take the opportunity to make your own view clear.