Sir Alex Younger, who left the UK’s foreign intelligence service last year, said he did not question the broader strategy of withdrawal, which he described as “inevitable”, but was critical of how it was done.
He also said having no physical presence in Afghanistan is a “blow” to intelligence networks.
Speaking on Times Radio, Sir Alex said: “The execution was unnecessarily self-harming.”
He added: “I would just say a couple of things.
“One, the idea that an abrupt departure is the same thing as a clean break seems to me at best naive and at worst wildly reckless, and I frankly can’t get my head around it.
“And I think [former US president Donald] Trump’s administration also does have a degree of responsibility, where unaccountably they began their negotiations with the Taliban two years ago with the relinquishment of our most important bargaining chip, which was our presence in Afghanistan, by setting a time for our departure.”
Sir Alex told the programme: “So I think that this has been unnecessarily messy and I hesitate to say that because as a practitioner I used to get really quite exasperated by people, armchair commentators, employing hindsight, but it is difficult to account for some of the decisions that have been taken.”
Asked about future ability to control terror from Afghanistan and the wider region without a physical presence, Sir Alex said: “It’s a blow, absolutely, and there is really no substitute for being on the ground and being able to generate the intelligence upstream, which we need to protect ourselves back here in the UK downstream.”
He went on to point out that al Qaida and Daesh, or so-called Islamic State, are “significantly degraded” organisations.
Sir Alex said disengagement and “turning our back” on a place like Afghanistan “leads inexorably to an increase in the threat to our country”.
He said that is what the West did in 1989, adding: “After the geopolitical struggle against the Soviet Union, which was essentially won, we turned our backs on Afghanistan, and the results are there for everybody to see.
“However hard it is, however humiliated we might feel, we mustn’t make that mistake again.”