The near-surface mounted (NSM) fibre-reinforced polymer (FRP) strengthening technique has become a viable strengthening method for reinforced concrete (RC) beams. Premature debonding failure of the NSM FRP strips is a common failure mode though that severely limits the effectiveness of the FRP intervention that results in reduced beam strength and deformability. The application of FRP U-jackets near the ends of the NSM strips offers a potential solution to the debonding problem. This paper presents the first systematic experimental investigation on the use of FRP U-jackets for delaying or preventing debonding failure in RC beams strengthened in flexure with tension face NSM FRP strips. A total of 11 large-scale RC beams were tested, with the investigated parameters being (i) arrangement of NSM carbon FRP (CFRP) strips, (ii) U-jacket material type, (iii) layout, width and inclination angle of FRP U-jackets. Failure modes, load-deflection and strain distribution responses of all tested beams were recorded. The test results bring clarity and understanding to the tested parameters and they also demonstrate the effectiveness of the U-jacket intervention.