Douglas Nicoll: Enigma codebreaker at Bletchley Park who warned about gaps in intelligence prior to the Falklands War

The Thatcher government's embarrassment at being caught by surprise when Argentina invaded the Falkland Islands on 2 April 1982 was all the greater because of a secret report that had been presented to the Joint Intelligence Committee 29 days before. In it the author, Douglas Nicoll, a former deputy director of GCHQ and wartime Bletchley Park Enigma codebreaker, described with exactitude a mistake that Britain's spymasters had repeatedly made in the past, and that a few weeks later would once again be seen to have rendered them blind.

Nicoll demonstrated to the JIC, when he presented his report on 4 March 1982, how they had missed the clues when Egypt invaded Israel in October 1973, and how they had been nonplussed in 1980 when Iraq invaded Iran – but his advice was spurned. The JIC considered itself “alert to the lessons to be learnt”, and decided that there was no need to act on Nicoll's recommendation that it should employ more staff to examine potential threats.

A month later, as the Royal Navy Task Force, led by Admiral Sandy Woodward in the aircraft carrier HMS Hermes, steamed to battle in the South Atlantic, a shamefaced Cabinet Office, of which the JIC is a part, and which had commissioned the original report, begged Nicoll to compile a second, using the enemy's very radio signals that indicated that the war was for real.

This second report by Nicoll magnified interest in the conclusions of his first – and, more than 20 years later, Professor Sir Lawrence Freedman, in The Official History of the Falklands Campaign (2005), referred to them extensively in the context of the six-week war that the JIC should have foreseen.

The crucial intelligence stumbling-block was called by Nicoll “perseveration”, and his explanation of this is known only because another historian, Professor Michael Goodman, used the Freedom of Information Act 2000 to winkle out enough of the still-classified report to include extracts of it in a book he edited with Robert Dover called Learning from the Secret Past.

In these published extracts, Nicoll describes perseveration as “the psychological phenomenon whereby certain data, such as telephone numbers and the spelling of names, if learned incorrectly the first time, are very difficult subsequently to learn correctly.” In other words: the JIC in a developing crisis tended to make up its mind very early and either not change it, or do so only when faced with very strong military evidence, so that vital last-minute signals of the enemy's true intentions were not picked up.

Nicoll's demand for more intelligence assessment staff – which would not be answered for another 20 years, until after the invasion of Iraq and the Butler Review of 2004 – recalls the effort of his old Bletchley Park chief, Sir Stuart Milner-Barry, to get the government to give Bletchley more staff and equipment in October 1941, which, because of its seeming impertinence, has gone down in history.

Nicoll was at that time a fresh-faced new entrant to Bletchley's Hut 6, straight out of St John's College, Oxford, where he had been studying classics. Hut 6 – considered the most valuable single source of intelligence for British and Allied land and air operations – was then still under the mathematician Gordon Welchman; but Milner-Barry, a world-class chess player, was soon to take over in leading its work on decoding German air force and army Enigma signals.

Milner-Barry personally took a letter that he, Welchman, and the mathematicians Alan Turing and Hugh Alexander had signed, to the door of No 10 Downing Street – and elicited from Winston Churchill, scrawled upon the paper, the reply for which they had hardly dared hope: “Action this day!” Nicoll stayed at Bletchley until the war's end in 1945.

Britain, as it drifted into industrial strife after the Second World War, was a disconcerting place for Nicoll. The London Post Office sorting clerk's son felt that this was not what he had been working for during the tough shifts tussling with the enemy's near-uncrackable codes. He and his wife, Winifred, a coach painter's daughter, who had also worked at Bletchley as a secretary in the administrative Hut 9, considered Britain by the 1970s to be descending into communism.

The couple had met not at Bletchley Park, but in Washington DC in 1947, where Bletchley's successor, GCHQ, had sent them to prepare British and US daily intelligence co-operation before the setting up of the US National Security Agency in 1952.

They married at St Alban's Church, Washington DC, in 1949, and were to have two sons. When GCHQ moved in the 1950s from Eastcote, west London, to Cheltenham, Gloucestershire, they followed it, before Nicoll was posted to Melbourne to work from 1963 to 1966 with Australian intelligence. On returning to Cheltenham, he became head of GCHQ Z Branch, distributing finished intelligence, then deputy director under Sir Brian Tovey. On his retirement in 1980 he was appointed CB.

The controversy over the September 2002 “dodgy dossier” and the 2003 invasion of Iraq stung him into emerging from retirement to protest at what he saw as the Blair government's disdain of the intelligence services in favour of its own political advisers and the “propaganda service” of Alastair Campbell.

“The management of the United Kingdom under Mr Blair and Mr Campbell has looked more and more like the direction of the USSR before 1989, wherein pro-government propaganda was so interlaced with official pronouncements that truth was impossible to find,” Nicoll declared in a letter published in a newspaper on 1 July 2003.

In February the following year he wrote to a newspaper in support of the Defence Intelligence Staff scientific analyst Dr Brian Jones, who had told the 2003 Hutton inquiry of his dismay over the September dossier.

Winifred died in 1987, and Nicoll remarried in 1992. His second wife, Cathryn Sansom, survives him, as do his two sons.

Douglas Robertson Nicoll, codebreaker and civil servant: born Wembley 12 May 1920; CB 1980; married first 1949 Winifred Campion (died 1987; two sons), second 1992 Cathryn Sansom; died Little Wymondley, Hertfordshire 29 September 2015.