After nearly 40 years as an expert on the Nazi era, the academic was ridiculed after wrongly authenticating the infamous Hitler diaries.
Lord Dacre is understood to have died on Sunday morning at Sobell House, a hospice in Oxford.
He had been suffering from cancer and is survived by three step-children.
Along with AJP Taylor, Lord Dacre was one of the most-respected historians of the modern era.
But his reputation was seriously undermined when he backed the Hitler Diaries in April 1983.
Both the German magazine Stern and Britain's Sunday Times were humiliated when it became apparent that they had paid millions for a hoax.
The 60 volumes, supposedly the personal thoughts of the dead dictator, were in fact the work of a German fraudster.
There had been a great deal of initial scepticism, with many won over by Lord Dacre's backing.
In reality, the diaries were made of paper, ink and glue of post-war origin.
Personal touch
The text was also peppered with historical inaccuracies and anachronisms.
Forger Konrad Kujau was jailed in Germany for four-and-a-half-years for the scam.
Kujau had based his work on a book called Hitler's Speeches and Proclamations compiled by a Nazi federal archivist.
He had added banal comments such as "Must get tickets for the Olympic Games for Eva" to give the work a personal touch.
Lord Dacre was also a director of Times Newspapers from 1974-1988.
Commercial acclaim
His first venture into writing about the Third Reich came after he was asked to go to Germany and investigate the end of Hitler's life.
His findings were published as The Last Days of Hitler to critical and commercial acclaim in 1947.
He also wrote Hitler's Table Talk (1953) and edited the books, Hitler's War Directives, 1939-45 (1964) and The Goebbels Diaries (1978).
His work had a popular touch and he paved the way for the modern television historian, appearing on programmes regularly and also working as a journalist.
In what is now a familiar tack, his academic critics complained he did not devote enough time to actually writing history.
Glittering academic career
Born Hugh Redwald Trevor-Roper, on 15 January 1914, he was educated at Charterhouse, then Christ Church, Oxford, becoming a research fellow of Merton College between 1937-39.
He was Regius Professor of Modern History and a fellow of Oriel College between 1957-80 when he was made an Honorary Fellow.
He became a Conservative Life Peer in 1979, taking the title of Baron Glanton in the County of Northumberland - near the family's home.
In 1957 he married Lady Alexandra Howard-Johnston, the daughter of the late Field-Marshal Earl Haig.
He became the step-father of her three children, and is survived by James Howard-Johnston, a lecturer in Byzantine history at Oxford University, Zenia Dennen and Peter Howard-Johnston. His wife died in 1997.