'High priest of deterrence'

Britain's leading defence civil servant during the last days of the cold war and an admirer of St Thomas Aquinas, who went on to be a vocal opponent of the allied invasion of Iraq.

Sir Michael Quinlan was instrumental in establishing Britain's Trident nuclear missile system.

Sir Michael Quinlan was regarded as one of the most brilliant and able civil servants of his generation. From 1988 until 1992, as the cold war ended and the Soviet Union collapsed, he was permanent under-secretary of state at the ministry of defence, the most senior non-political civilian position in Britain. From the 1970s he had been at the centre of policy and decision-making on Britain's defence policy, especially the issue of nuclear deterrence. In fact he became known as "the high priest of deterrence". He was also was instrumental in the establishment of the Trident nuclear missile system.

In 2003, after his retirement from the civil service, he became a formidable and eloquent opponent of the Iraq war. He argued that for a war to be morally justifiable it had to pass a series of tests, which were founded on the teachings of St Thomas Aquinas. The criteria were: just cause, proportionate cause, right intention, right authority, the reasonable prospect of success and war as last resort.

He said in a speech in 2004, "there is nothing in the approach that need be exclusive to Christianity, and there are bodies of ethical reasoning of much the same basic character alive in Judaism and in Islam, and indeed in secular humanism? [Yet] there are elements of the Geneva Conventions that could almost have come straight from Aquinas. So this tradition is, I believe, the best basis for approaching a general ethic of war, and the best framework for considering whether we need to develop our ethic further, or to modify aspects of it."

In 2007 this philosophy was encapsulated in Just War - The Just War Tradition: Ethics in Modern Warfare, a book he wrote with the former British chief of the defence staff, Lord Guthrie of Craigiebank. While he was certain Saddam Hussein was not behind al Qa'eda's September 11 attack on the United States, Quinlan conceded that Saddam had to be confronted for his "intolerable" defiance of the UN, but "we did not have to be straitjacketed into choosing between inaction and the extraordinary step - unique, for the West - of starting from cold a full-scale war".

Quinlan was born into a Roman Catholic family in Middlesex, the second of five children of an accounts clerk. He was educated by the Jesuits at Wimbledon College and Merton College, Oxford, where he was awarded a double first in classics. He was the first in his family to go to university. After national service in the RAF, he joined the air ministry. One of his first postings, in 1956, was as private secretary to Winston Churchill's son-in-law, Christopher Soames, then a junior air minister.

After another four years as private secretary to two chiefs of the air staff, he held a series of significant defence policy posts. He advised on arms control issues and worked with Nato. A number of governments recognised his exceptional abilities and he worked at the Cabinet Office on devolution issues, at the Treasury, and was then employment permanent secretary before returning to defence in 1988.

At the ministry he was known alternatively as "Big Q" and as "the Jesuit" for his devout Catholicism and his sharpness. One MP observed of him: "Quinlan believed it the duty of civil servants to proffer unpalatable advice to politicians, if they thought it was in Britain's interest." He was appointed Knight Grand Cross of the Order of the Bath in 1991. For five years after his retirement he was director of the prestigious international forum, the Ditchley Foundation. He also became a trustee of the Science Museum, the chairman of the liberal Catholic weekly, The Tablet, and the director of Lloyds Bank and TSB (until 1998) and Pilkington. His book Thinking About Nuclear Weapons: Principles, Problems, Prospects was published only weeks before his death.

Born Aug 11, 1930. Died Feb 26, 2009. * The National