A formwork for concreting the inner lining of tunnels, made up of a series of panels forming the vault, the sides and the corners of the base of the tunnel, assembled on a carriage (c) which moves the formwork along the longitudinal axis of the tunnel (t), from one course to the next, supported on railway tracks placed previously on the footings/inverts of the tunnel, said formwork panels being provided with articulated connections which allow collapsing the panels forming the small side walls and the sides of the tunnel for stripping and being able to move the formwork to the next placement, before placing it and the subsequent concreting of the corresponding course, wherein the said formwork comprises, from the inside out: a) a resistant structure formed by at least two polygonal rings, perpendicular to the axis of the tunnel, made up of a variable number of ribs which are articulated to one another by the outer side edges, which are located closer to the wall of the tunnel, while the two inner edges of two annexed ribs are related to one another by a telescopic prop which allows varying the angle formed by the two consecutive ribs between which it is located, and consequently varying the curvature of the perimeter ring in each sector of the tunnel, such that by also varying the number of ribs it is possible to form a polygonal ring, inside the formwork, adaptable to any measurement and configuration of the tunnel (t) to be constructed; characterised in that, from the inside out, it further comprises: b) a series of intermediate parts, or secondary externally defining a curvature in accordance with each area or sector of tunnel in which it is assembled, supported on the rib or ribs of the polygonal rings which structure the formwork and makes up the supporting means for the sheet metal defining the skin of the formwork; c) a series of flexible steel sheets, each of which is assembled or formed with the radius of curvature suitable for the area or sector of tunnel to be constructed, making up the skin of the formwork on a surface having the length of the formwork and the width corresponding to a sector of tunnel, defined by one of the intermediate parts supporting it and by the rib or ribs forming the inner structural rings. a) a resistant structure formed by at least two polygonal rings, perpendicular to the axis of the tunnel, made up of a variable number of ribs which are articulated to one another by the outer side edges, which are located closer to the wall of the tunnel, while the two inner edges of two annexed ribs are related to one another by a telescopic prop which allows varying the angle formed by the two consecutive ribs between which it is located, and consequently varying the curvature of the perimeter ring in each sector of the tunnel, such that by also varying the number of ribs it is possible to form a polygonal ring, inside the formwork, adaptable to any measurement and configuration of the tunnel (t) to be constructed; characterised in that, from the inside out, it further comprises: b) a series of intermediate parts, or secondary externally defining a curvature in accordance with each area or sector of tunnel in which it is assembled, supported on the rib or ribs of the polygonal rings which structure the formwork and makes up the supporting means for the sheet metal defining the skin of the formwork; c) a series of flexible steel sheets, each of which is assembled or formed with the radius of curvature suitable for the area or sector of tunnel to be constructed, making up the skin of the formwork on a surface having the length of the formwork and the width corresponding to a sector of tunnel, defined by one of the intermediate parts supporting it and by the rib or ribs forming the inner structural rings.