Allen, who had not played since his agonising World Championship semi-final loss to Wu Yize in early May, lost 4-2 to Brown in the all-Northern Ireland first-round contest in Belfast.

Former world number one Allen becomes the 10th top-16 player to fall at the first hurdle this year, with Ronnie O'Sullivan, Judd Trump and Mark Selby among the early casualties.

Allen, 40, took the opening frame on Tuesday afternoon but 39-year-old Brown hit back by taking the next two.

While Allen levelled the match at 2-2, Brown built breaks of 53 and 80 in the final two frames to progress.

Brown will now aim to advance further than last year, when he lost 5-2 to Allen in the quarter-finals.

Defending champion Jack Lisowski is in action against Anthony McGill during Tuesday's evening session (21:00 BST).