John Albert Dellow was born in London in 1931 with policing in his blood. His father was a PC in the City of London force. Educated at William Ellis School, Highgate, and the RGS, he completed his National Service in the RAOC, worked for Shell and flirted with architecture before joining his father’s force in 1951.

Dellow transferred to the Manchester City force before becoming a superintendent and chief superintendent in Kent. His potential spotted, he became an assistant chief constable in 1969 and in 1973 came to London as one of a small but growing band of provincial officers brought into the Yard to introduce new blood.

Metropolitan Police chief who won praise during the Iranian embassy siege and tightened up security after ‘the Palace intruder’

It was May 5, 1980, almost a week after the six young men, trained and equipped by an Iraqi intelligence officer, had rushed into the embassy overlooking Hyde Park and captured 26 hostages, including PC Trevor Lock, the embassy’s police guard.

At first they had demanded that Iran agree to the creation of a breakaway Arab state as well as the release of political prisoners. Now the demands had been reduced to a safe passage out of Britain. Margaret Thatcher, the prime minister, remained steadfast. There would be no “surrender to terrorism”.

Sitting in the nursery school close to the embassy, which the police had turned into a forward base, John Dellow, the deputy assistant commissioner, still hoped that the gunmen and a score of hostages would walk out unscathed. Others had already been released.

Twice in the 1970s, Scotland Yard had brought sieges involving an IRA team at Balcombe Street and armed robbers in a Spaghetti House restaurant to a peaceful conclusion. This time Dellow knew things could be different.

The siege, as he later wrote, was unique. The terrorists had planned the attack and were well trained. There was also an unpredictable element of martyrdom at play and, sure enough, tension was building by the Sunday afternoon. Shots were heard and then the body of an Iranian diplomat was dumped on the pavement. Dellow feared more murders from the increasingly desperate and suspicious terrorists. The time had come for action. At 7.07pm Dellow reached for a sheet torn from a Yard notebook and signed the transfer of control of the siege to a waiting team of SAS soldiers.

Within minutes the air rang to the sound of stun and smoke grenades and the crackle of gunfire. Hidden television cameras broadcast Operation Nimrod live to the world.

Dellow later said he had no regrets about sending in the SAS. Five of the gunmen were killed plus one of the hostages but 20 others survived: the SAS had forecast a far higher death toll.

Dellow said he had no regrets about sending in the SAS during Iranian embassy siege in May 1980

While many of the post-mortems on the siege dwelt on the success of the SAS, and the way co-operation had worked across government, Dellow, in a study six years later, added a prescient footnote about the future. The TV coverage of the siege had shown the increasing reach of technologies which might one day, he warned, become a weapon for the terrorists themselves.

When the Iranian embassy siege began, he oversaw and co-ordinated the police operation. He had the advantage that he had been the first British police officer to attend the Joint Services Defence College. This made for a good rapport with both the SAS commander, who was on hand from the start of the siege, and Whitehall officials.

Dellow’s letter authorising control of the embassy incident to the SAS

Two years after the triumph of the Iranian embassy operation, the Yard faced one of its biggest disasters. Michael Fagan, a small-time crook with mental health problems, broke into Buckingham Palace, bypassed security and reached the Queen’s bedroom. Dellow, now an assistant commissioner, investigated the police inadequacies. Fagan had in fact climbed into the palace twice, the first time drinking half a bottle of wine before leaving. The second time he reached the Queen’s bedroom. Confronted by him, she had difficulty in raising the alarm or finding help.

No punches were pulled in Dellow’s report, which revealed a catalogue of incompetence. Fagan triggered alarms on both intrusions, but they were ignored or discounted. One commander resigned and another senior officer was transferred. Royal protection was overhauled, meanwhile, ending years of laissez-faire policy.

Dellow himself moved on to the Yard department covering support branches such as fingerprints, traffic patrols and communications. In 1984 he took control of the 3,400-strong C department, CID, which needed an experienced and competent hand.

Once again he found himself in the heart of a controversy overseeing the hunt for the gang behind the £26 million Brinks-Mat bullion robbery. When DC John Fordham was stabbed to death during a surveillance operation at the home of a suspected launderer Kenneth Noye, Dellow publically refuted criticisms of the operation.

Lord Wasserman wrote. Sir John Dellow made a large contribution to introducing science and technology to UK policing.

In 1983, when I returned to the Home Office from the Cabinet Office to take charge of the department’s work on providing scientific and technological support to police forces across the UK, one of my first tasks was to chair a meeting of senior chief constables called to demonstrate a piece of kit which my team had developed called an automatic number plate reader (ANPR). The demonstration was received in almost total silence, followed by a number of extremely disobliging comments from chief constables about how much time and money must have been wasted building this useless “gismo”.

The mood in the room changed completely, however, when John Dellow, representing the Met, said that he thought that this piece of equipment might be very useful and that he was prepared to try it on the streets of London. This same positive attitude to technology and innovation marked the whole of his career as a senior officer and, more importantly, his time as president of the Association of Chief Police Officers.

Sir John Dellow, police chief, was born 5 June, 1931. He died on 30 December, 2022, aged 91.