The use of fibre-reinforced polymer (FRP) sheets and laminates for strengthening concrete structures is established in both literature and practice as an effective method of retrofitting. Due to environmental concerns, there has been a growing interest in using natural FRPs as a more sustainable alternative to synthetic ones. In this study, a flax fibre-greenpoxy composite plate was designed and applied to upgrade both undamaged and pre-damaged reinforced concrete (RC) beams. The biocomposite plate was fabricated with 40% fibre content by mass using the hand lay-up technique. A four-point bending test was conducted on 1840 mm long steel-reinforced concrete beams. The pre-damaged beams were preloaded until the tension steel yielded to simulate service conditions before strengthening. The unstrengthened, undamaged and pre-damaged beams were tested to determine the effectiveness of the material as a retrofitting solution. The results show that the developed natural FRP plate enhanced the load-carrying capacity of the undamaged and pre-damaged beam sets by 43.35% and 65.02% respectively. Their application in the building industry can support the transition towards environmentally responsible construction practices.