Mr Ryder, 47, who became an MP in 1983, was John Major's Chief Whip from 1990 until he stood down in last summer's reshuffle. He said in a statement yesterday: "I have decided, after long consideration, to inform the Mid- Norfolk Conservative Association that, with much sadness, I shall not be a candidate at the next general election."
He said he would carry on with his constituency duties and offered his "total support" to the Prime Minister.
Mr Ryder gave no hint as to his reason for going, but even when he was Chief Whip he was said to be considering giving up politics for an academic career. He is also reported to suffer from a bad back.
Mr Major appointed the self-effacing Mr Ryder Chief Whip after reaching 10 Downing Street in November 1990. The job involves getting the Government's legislative programme through Parliament.
As Mr Major's Commons majority fell, and backbench Tory MPs became more rebellious, Mr Ryder's job became harder. He was blamed for the failure of the Government to win the crucial Commons vote over VAT on fuel.
Conservative Central Office said Mr Ryder is their 52nd MP to announce that he or she will stand down at the election.Reuse content