A flexible door comprises two metal posts (1) connected by an upper cross-member (2) in which a winding roller is housed. The movable closure element of the door is constituted by a flexible sheet (3) of plastics material having at least one transverse reinforcing bar (4). The side edges of the flexible sheet (3) slide in vertical guides (5) carried by the two posts (1) and each comprising two facing flexible vertical strips (6). Each strip (6) is constituted by at least one layer of fabric made of synthetic fibres which has a high transverse stiffness much greater than its longitudinal stiffness. The outer longitudinal edge of each strip (6) is fixed to the respective metal post (1) and the strip (6) projects inwardly from the post (1), relative to the door opening. In the event of a knock, the guide (5) deforms resiliently to allow the flexible sheet (3) to come out, the sheet (3) subsequently going back into the guide automatically.