The invention relates to a formwork element which is specially designed for the construction of walls and similar. The formwork element comprises a hollow rectangular primatic block or body (1') which is open at the lower and upper faces thereof and which is made from expanded polystyrene foam or another similar material. The edges of the aforementioned faces are provided with numerous small rectangular prismatic projections (3) in the form of cubes which define therebetween similarly-configured housings (4), the projections on the upper face being offset in relation to those on the lower face. In this way, when the bodies are stacked such that they are offset lengthways, a tongue-and-groove connection is created therebetween, ensuring optimum stability. Numerous bodies (1') can be stacked to produce a level chamber which is closed at the inner and outer faces thereof and which can receive a mass of reinforced concrete forming the resistant element of the wall. According to the invention, the vertical reinforcements pass through openings (6) in the smaller lateral walls (5) thereof.