He edited the complete works of Walter Bagehot, revered as the greatest expert on the constitution; his own books included “Life, Death and the Law: Law and Christian Morals in England and the United States,’’ “Agonizing Choice: Birth Control, Religion and the Law,’’ and “Pope John Paul II: His Travels and Mission.’’ He also served as adviser on arts, cultural, and constitutional matters to British Sky Broadcasting chairman James Murdoch and chief executive Jeremy Darroch.

Mr. St John-Stevas was elected to the House of Commons in 1964 and remained a member until 1987.

David Hughes, chief editorial writer for The Daily Telegraph, said he once went to Mr. St John-Stevas’s home to do an interview and was greeted by his host in “a full-length velvet dressing gown and a cardinal’s cap.’’

In a House of Lords debate in June, Mr. St John-Stevas digressed from the topic to discuss fashion, confessing a weakness for dressing well. He recalled that he once begged to leave early from a Cabinet meeting to prepare for an evening function.

Thatcher, he said, responded that she was going to the same function. His riposte? “But Prime Minister, it takes me much longer to change than it does you.’’