A leak detection system, comprising: a flexible pipe structure having an internal reinforcement layer, a fluid isolation layer providing a first barrier to a first fluid inside the flexible pipe structure, and an outer protection shell providing a second barrier to a second fluid outside the flexible pipe structure, the first barrier and the second barrier defining a space therebetween in which external reinforcement layers (3, 4) are located, one or both of the internal reinforcement layer and the external reinforcement layers (3, 4) being made of metal, characterised in that: the system comprises at least one sensor (6A, 6B) comprising an optical fiber imbedded or placed in close proximity to a material which has at least one property that changes in the presence of liquid or gas, placed in the space; the material is configured such that the at least one property undergoes a change in the presence of the first fluid or the second fluid in the space between the first and second barriers due to a leak in the first barrier or the second barrier, and the sensor (6A, 6B) is sensitive to the change of the at least one property of the material such as to detect the change; and the role of the material is performed by the fluid isolation layer or by one of the external reinforcement layers (3, 4) or by the outer protection shell. a flexible pipe structure having an internal reinforcement layer, a fluid isolation layer providing a first barrier to a first fluid inside the flexible pipe structure, and an outer protection shell providing a second barrier to a second fluid outside the flexible pipe structure, the first barrier and the second barrier defining a space therebetween in which external reinforcement layers (3, 4) are located, one or both of the internal reinforcement layer and the external reinforcement layers (3, 4) being made of metal, characterised in that: the system comprises at least one sensor (6A, 6B) comprising an optical fiber imbedded or placed in close proximity to a material which has at least one property that changes in the presence of liquid or gas, placed in the space; the material is configured such that the at least one property undergoes a change in the presence of the first fluid or the second fluid in the space between the first and second barriers due to a leak in the first barrier or the second barrier, and the sensor (6A, 6B) is sensitive to the change of the at least one property of the material such as to detect the change; and the role of the material is performed by the fluid isolation layer or by one of the external reinforcement layers (3, 4) or by the outer protection shell. the system comprises at least one sensor (6A, 6B) comprising an optical fiber imbedded or placed in close proximity to a material which has at least one property that changes in the presence of liquid or gas, placed in the space; the material is configured such that the at least one property undergoes a change in the presence of the first fluid or the second fluid in the space between the first and second barriers due to a leak in the first barrier or the second barrier, and the sensor (6A, 6B) is sensitive to the change of the at least one property of the material such as to detect the change; and the role of the material is performed by the fluid isolation layer or by one of the external reinforcement layers (3, 4) or by the outer protection shell.