52-year-old Lord Strathclyde told Cameron in a letter that he wanted to return to his career in the private sector and "take up other threads of my life and other interests."
According to his friends, there is absolutely no political reason for his departure.
It is 25 years since Margaret Thatcher gave him his first job as a whip and he has been on the Conservative front bench ever since. He has served six party leaders, including three prime ministers.
The peer has been the Tory leader in the Lords for 14 years since his predecessor Lord Cranborne was sacked by William Hague in 1998 for "running in like an ill-trained spaniel" and negotiating secretly with Tony Blair over Lords' reform.
He was also a minister of state at the Department of Trade and Industry during the 1990s and his various junior ministerial jobs covered tourism, Scotland, the environment, and consumer affairs.
He will be replaced by Education Minister Lord Hill of Oareford, who entered the House of Lords in 2010.
London: Lord Strathclyde, the leader of UK's House of Lords and a close friend of Prime Minister David Cameron, today resigned from the Cabinet with immediate effect to pursue "other interests" in his life.
First Published: Monday, January 07, 2013, 20:35