An actuation system for actuating an intake valve associated with a cylinder in an internal combustion engine, comprising: - a master piston controlled, directly or indirectly, by a cam of a camshaft of the internal combustion engine, - a slave piston, which actuates said intake valve and which is hydraulically controlled by said master piston, by means of a volume of pressurized fluid interposed between the master piston and the slave piston. - wherein the intake valve is biassed by at least one spring towards a closed position, and - a control valve, which controls the communication between said volume of pressurized fluid and an environment at a lower pressure connected to a fluid accumulator, - in such a way that: - when the control valve keeps said communication closed, the intake valve is sensitive to the movement of said first cam, while - when the control valve opens said communication, fluid is discharged from the volume of pressurized fluid into said environment at lower pressure, so that the intake valve is biassed towards the closed position by said spring and remains insensitive to the movement of said first cam. - said system being characterized in that the control valve is mechanically controlled by a second cam which is carried by said camshaft and which has the sole function of controlling the control valve, and in that the second cam is configured and angularly positioned on said camshaft in such a way as to cause the opening of the control valve, and the resulting sudden closing of the intake valve, in advance of the closure of the intake valve which would be determined by said first cam. - a master piston controlled, directly or indirectly, by a cam of a camshaft of the internal combustion engine, - a slave piston, which actuates said intake valve and which is hydraulically controlled by said master piston, by means of a volume of pressurized fluid interposed between the master piston and the slave piston. - wherein the intake valve is biassed by at least one spring towards a closed position, and - a control valve, which controls the communication between said volume of pressurized fluid and an environment at a lower pressure connected to a fluid accumulator, - in such a way that: - when the control valve keeps said communication closed, the intake valve is sensitive to the movement of said first cam, while - when the control valve opens said communication, fluid is discharged from the volume of pressurized fluid into said environment at lower pressure, so that the intake valve is biassed towards the closed position by said spring and remains insensitive to the movement of said first cam. - said system being characterized in that the control valve is mechanically controlled by a second cam which is carried by said camshaft and which has the sole function of controlling the control valve, and in that the second cam is configured and angularly positioned on said camshaft in such a way as to cause the opening of the control valve, and the resulting sudden closing of the intake valve, in advance of the closure of the intake valve which would be determined by said first cam. - when the control valve keeps said communication closed, the intake valve is sensitive to the movement of said first cam, while - when the control valve opens said communication, fluid is discharged from the volume of pressurized fluid into said environment at lower pressure, so that the intake valve is biassed towards the closed position by said spring and remains insensitive to the movement of said first cam. - said system being characterized in that the control valve is mechanically controlled by a second cam which is carried by said camshaft and which has the sole function of controlling the control valve, and in that the second cam is configured and angularly positioned on said camshaft in such a way as to cause the opening of the control valve, and the resulting sudden closing of the intake valve, in advance of the closure of the intake valve which would be determined by said first cam.