Tesla chief executive Elon Musk has admitted that developing self-driving technology is harder than he first thought.

The admission came on Twitter over the weekend, when Musk replied to a Tesla owner who had changed the name of their vehicle to 'Two Weeks' – a reference to Musk's promise that an updated version of Autopilot would arrive within that time frame. That deadline has since been missed.

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Musk replied to gentle trolling: "Haha, FSD [full self-driving] 9 beta is shipping soon, I swear! Generalized self-driving is a hard problem, as it requires solving a large part of real-world AI. Didn't expect it to be so hard, but the difficulty is obvious in retrospect."

Musk added: "Nothing has more degrees of freedom than reality."

The Tesla boss first promised one of his cars would be able to drive itself across the US without human help in 2017. But that target came and went, and four years later the goal is still yet to be realized.

As for Tesla's full self-driving system, which is a $10,000 upgrade on the regular Autopilot system fitted to Tesla cars, this was supposed to provide full automation in 2018, then 2019. Although it has improved over the years, a truly driverless Tesla available to the public still doesn't exist.

Autopilot is due to receive a major update with the release of the FSD 9 beta, with it being the first installment to incorporate the new Tesla Vision system. This relies entirely on cameras and AI-powered computer vision instead of a combination of radars and cameras, as used by the driver assistance systems of other automakers.

If Tesla can eventually deliver on Musk's promise of an Autopilot system that no longer requires human intervention, or even supervision, then the next step could be to turn Teslas into a fleet of autonomous taxis, earning money for their owners while they sleep or are at the office – something Musk promised would be ready by 2020.