The present invention relates to a linking device (2) arranged between two insulating panels (1a, 1b) and used in lost formwork in civil engineering. Since the linking device is foldable, it allows separating the two panels (1a, 1b) from each other for holding the fluid structural material, such as concrete, in the construction of walls and partitions, and placing the two panels (1a, 1b) in contact with each other, when the device is folded, to make storage and transport easier. The linking device (2) consists of a central piece that ends in curved hooks having an opening and two ends (11a, 11b) each having an outer elongated portion embedded in one of the panels of insulating material, and an inner portion joined to the central piece, which comprises a longitudinal axis (10) that rotates through 90° between the two stated positions (13, 14). The formwork block comprising this device is produced in a single piece, since the device can be placed in the mould where the insulating panels are produced by injection-moulding.