An internal combustion engine comprising: - a crankcase, - an oil sump, - at least one balancing shaft rotatably mounted about an axis in a balancing tunnel formed in the crankcase and driven in rotation at high speed, wherein the balancing tunnel has a first end portion, a second end portion and a central portion extending between said first and second end portions (22a, 22b), wherein the first and second end portions (22a, 22b) of the balancing tunnel carry respective bearings (24a, 24b) which support the balancing shaft in a rotatable manner about said axis, and wherein the balancing shaft has an eccentric section at least at the central portion of the balancing tunnel, - a blow-by gas recirculation system having an inlet channel in communication with the oil sump, an outlet channel in communication with an intake manifold, a centrifugal separator arranged between said inlet channel and said outlet channel, and at least one drainage channel which opens into the oil sump for discharging the separated liquid, wherein: - said inlet channel of the blow-by gas recirculation system communicates with said first end portion of the balancing tunnel, - said centrifugal separator is housed in said second end portion of the balancing tunnel and is driven in rotation by said balancing shaft, and - said outlet channel of the blow-by gas recirculation system communicates with a gas outlet of said centrifugal separator, whereby, during operation, the flow of blow-by gas flows successively through the central portion of the balancing tunnel and through the centrifugal separator in the path from the inlet channel to the outlet channel, such that the high speed of rotation of the balancing shaft and the eccentric section of the balancing shaft at the central portion of the balancing tunnel impart a whirling path to the flow of blow-by gas in the central portion of the balancing tunnel which carries out a preliminary centrifugal separation of the oil from the gas upstream of the main centrifugal separation performed by the centrifugal separator. - a crankcase, - an oil sump, - at least one balancing shaft rotatably mounted about an axis in a balancing tunnel formed in the crankcase and driven in rotation at high speed, wherein the balancing tunnel has a first end portion, a second end portion and a central portion extending between said first and second end portions (22a, 22b), wherein the first and second end portions (22a, 22b) of the balancing tunnel carry respective bearings (24a, 24b) which support the balancing shaft in a rotatable manner about said axis, and wherein the balancing shaft has an eccentric section at least at the central portion of the balancing tunnel, - a blow-by gas recirculation system having an inlet channel in communication with the oil sump, an outlet channel in communication with an intake manifold, a centrifugal separator arranged between said inlet channel and said outlet channel, and at least one drainage channel which opens into the oil sump for discharging the separated liquid, - said inlet channel of the blow-by gas recirculation system communicates with said first end portion of the balancing tunnel, - said centrifugal separator is housed in said second end portion of the balancing tunnel and is driven in rotation by said balancing shaft, and - said outlet channel of the blow-by gas recirculation system communicates with a gas outlet of said centrifugal separator,