A method and a structural element for the construction of a directionally flexible and watertight tunnel lining (1) or tunnel course (1), the method using pre-fabricated elements of concrete or sandwich structure (3, 4, 5; 73;73′). Interspace between such elements and extending transversely of the direction of the tunnel course or tunnel lining receives an interspace cast (14) using an outer flexible formwork (2) in the form of skirts (2′, 2″) attached to the outside of the elements (3, 4, 5; 73, 73′), and which through interconnection (11) thereof form a flexible, “bag-like” formwork (2) on the outside of the elements (3, 4, 5; 73, 73′). Together with conventional formwork (12) on the inside of the elements (3, 4, 5; 73, 73′) there is formed a defined cavity (13) which can be filled with injectable mass of concrete. After the concrete has hardened it will provide support to the installed elements (3, 4, 5; 73, 73′) as well as providing support relative to exposed surface portion of rock surface (10). The elements (3, 4, 5; 73, 73′) can be provided on the outside with a membrane (17). Further, the elements can be provided with injection paths (16) in order to at any time establish a completely watertight tunnel lining (1).