Although same-sex relationships were decriminalised in 1967, the intelligence agencies continued to bar LGBT+ entrants until 1991. Even after that date, Richard Moore pointed out, serving officers who had not disclosed their sexuality during the vetting process faced harsh treatment.
The ban on LGBT+ entrants, said Mr Moore, was based on "misguided view" that they would be more susceptible to blackmail than straight people by hostile states, making them a security risk.
Comment: The thing is, at the time this was likely true. Since being gay was stigmatized in the past, the threat of exposure was very real. If we look at it through today's lens, without putting it in the context of the time, it seems simply discriminatory. But put within the context of the time, it's not without merit, although the rule likely stayed in place longer than it should have.
"It meant that until 1991, being openly LGBT+ in MI6 would cause you to lose your job or prevent you from being allowed to join in the first place," said Mr Moore.
"Committed, talented, public-spirited people had their careers and lives blighted because it was argued that being LGBT+ was incompatible with being an intelligence professional. Because of this policy, other loyal and patriotic people had their dreams of serving their country in MI6 shattered."
"This was wrong, unjust and discriminatory. Today, I apologise on behalf of MI6 for the way our LGBT+ colleagues and fellow citizens were treated and express my regret to those whose lives were affected."
The apology by the head of the Secret Intelligence Service (SIS) is the latest in the defence and security establishment on discrimination experienced by members. The Ministry of Defence has announced medals stripped from those who were dismissed from the armed forces for being gay would be restored. Work has also begun on pensions they lost as result and the issue of the criminal records they sustained.
"I pay tribute to the extraordinary resilience and loyalty to Service and country of LGBT+ colleagues past and present who slowly turned the tide by educating their workmates and fighting for change. As well as apologising, I am thanking current and former LGBT+ colleagues for the contribution they have made, and continue to make, to MI6 and to our country", said Mr Moore.
The plaudits from LGBT+ groups for encouraging diversity were a dramatic change from the past persecutions. One of the most notorious examples was the treatment of Alan Turing, whose codebreaking was vital during the Second World War. He lost his security clearance after a conviction for indecency in 1952 and later took his own life.
A series of Cold War espionage cases involving LGBT+ individuals led to linking sexuality with betrayal in public minds. Two of the Cambridge Five spy ring, Guy Burgess and Anthony Blunt, were gay, while a third, Donald MacLean, was bisexual. All, however, had chosen to work for the Russians because of ideological reasons, rather than being blackmailed into doing so.
Criminalisation of homosexual acts did lead to blackmail in some cases such as that of John Vassall, a gay civil servant who spied for the Russians after being threatened with publicity. But there were plenty of "honeytrap" operations by intelligence services of many countries exploiting heterosexual affairs - including the most well publicised British security breach of the era involving sex, the Profumo scandal.
The media played its part in seeking to whip up witch-hunts. In one example from 1963, after the Vassall case, national tabloid the Sunday Mirror offered the intelligence and security services tips on "How to Spot a Possible Homo". The list included "his tie has the latest knot", "an unnaturally strong affection for his mother" and a "gay little wiggle".
Comment: If the barring of LGBT entrants was lifted 30 years ago, why apologize now?
See also: