A clamping system for connecting a first and a second tube whose opposite ends have clamping surfaces protruding from the cylindrical outer surface of the tubes. The system includes a collar having a belt able to be clamped around the clamping surfaces. The belt includes a first and a second flank between which an internal recess able to receive the clamping surfaces is delimited. The collar carries an external retaining device having a retaining bar, which protrudes axially from the first flank while being oriented transversely to the axis of the collar and which has a transverse retaining ridge, the bar being able, from a rest situation in which the retaining ridge projects towards the axis of the collar relative to the edge of the first flank in the unclamped situation of the collar, to be elastically deformed to space the retaining ridge away from the axis of the collar.