Mr Koussa flew from Tunisia, where he had been on a diplomatic mission, and landed at Farnborough airport before being shuttled to London for immediate talks with high-ranking Foreign Office officials.
Mr Koussa was one of the leading figures in Col Gaddafi's government and one of the architects of Libya's recent rehabilitation in the eyes of the international community.
As such, his departure represents a significant scalp both for the coalition against Col Gaddafi and the rebels seeking his overthrow, and will fuel speculation that more defections could follow.
A spokesman for the Foreign Office said: "We can confirm that Moussa Koussa arrived at Farnborough Airport on 30 March from Tunisia. He travelled here under his own free will. He has told us that he is resigning his post. We are discussing this with him and we will release further detail in due course."
He added that the senior politician's "role was to represent the regime internationally - something that he is no longer willing to do".>>>
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