As Sir Keith Joseph he held a number of senior Cabinet posts, including Industry Secretary and Education Secretary, but he is best remembered for his influence as a right-wing thinker.
Lady Thatcher paid tribute to him: "Today I have lost one of my dearest friends, England one of her greatest men.
"His best memorial lies in the younger generations of politicians whom he inspired. But for me he is irreplaceable."
Lord Joseph had been in poor health for several years and suffered a stroke last year.
He died yesterday morning of chest complications in the Royal Brompton Hospital in London. His wife and children were present.
Lord Tebbit said: "I would like to say how enormously sad I am that one of the most generous, kind and able politicians of our time has been lost to us . . . There are too few like him."
Jeremy Hanley, the Tory chairman, mourned the passing of "a gentle and noble man," who he said would be "greatly missed by all in the party".
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