An internal combustion engine has intake (20) and exhaust (22) poppet valves, a first and a second set of cam lobes (64) for operating the intake and exhaust poppet valves (20,22) respectively, and a third set of cam lobes (68) for producing an additional selectable valve event in order to allow the engine to operate as a compression brake. The third set of cam lobes (68) and at least one of the first and second set of cam lobes (64) are rotatable relative to one another and form part of an assembled camshaft (60) so as to be rotatable about a common axis. A phasing system (50) acts on the assembled camshaft (60) to allow the phase of the third set of cam lobes (68) to be changed relative to the engine crankshaft.