Mike Beaton reportedly spilled the beans to a flight attendant friend of his a few weeks ago after a wild night on the town in Johannesburg -- this during a brief grounded period he was enjoying before he was scheduled to fly again the following day.

In texts, obtained by The Sun, Beaton purportedly bragged about a hot hookup he experienced with a Welsh woman ... which was full of cocaine, booze and sex. Beaton wrote, "I’ve lost my shirt somewhere and one of the local lads produces a plate with a few lines of coke. So then there’s a debate about whose chest is the best to do a bump off."

He also proudly boasted ... "I've been a very naughty boy."

Crazy enough, Beaton tried boarding his flight the next day and was attempting to fly back to London ... but his friend ratted him out to their bosses, and they halted everything.

Beaton was forced to stay in Johannesburg for at least an extra day as they did drug testing -- which he reportedly failed at first. Once he was sober again, though, they shipped his ass back to the UK as a standard passenger ... and he got fired from BA shortly thereafter.

British Airways tells us, "Safety is always our top priority. The matter was referred to the CAA (Civil Aviation Authority) and this individual no longer works for us." BA brass was reportedly shocked by his behavior.