LONDON — Hugh Thomas, a British historian and associate of Prime Minister Margaret Thatcher whose magisterial work chronicled great upheaval in the Hispanic world, from Spain’s imperial expansion to its civil war, died here on Sunday. He was 85.

His son, Isambard, said the death was preceded by a stroke.

Mr. Thomas produced a broad canon of work, including novels and studies of the American slave trade, the history of Mexico, disarmament (he had worked in the British Foreign Office as a young man) and the beginnings of the Cold War.

But it was his vast explorations of Spain on which his reputation was built, beginning with “The Spanish Civil War” in 1961. Banned in Spain during the Franco era, it won the prestigious Somerset Maugham Prize in 1962 and is regarded as a classic, remaining in print, with several revisions, to this day.

“Thomas has long belonged to the elite of Spanish studies,” Publisher’s Weekly said in 2005. But while his assessment of the civil war seemed “strongly sympathetic” to the Republican side, the magazine said, he was portrayed in later years as shifting toward “an increasingly conservative vision of the Spanish past.”