Astley fell off the professional tour last year.
Astley, 32, fell off the professional tour last year but fired a break of 119 in the seventh frame en route to a 6-2 win.
Astley is the second amateur to beat a UK champion this week after Shaun Murphy was knocked out by China’s Si Jiahui on the opening day.
And the result is particularly embarrassing for Robertson after he concurred with Murphy’s controversial claim on Tuesday that amateurs did not deserve the right to play on the professional tour.
Astley, better known for his role in the snooker play ‘The Nap’ at the Crucible in 2016, was appearing as a ‘top-up’ player due to there not being enough professionals to complete the 128-player field.
He capitalised on a sluggish start by Robertson to edge into a 3-1 lead, then extended his advantage with nerveless 83 after Robertson jawed a red to the middle.
The Australian who won the title in thrilling fashion over Judd Trump last year, responded with a break of 124 to narrow the deficit to 4-2.
But Astley reeled off a 119 to go 5-2 up, before edging over the line to wrap up the best win of his career, and one of the tournament’s greatest upsets.