Method for applying labels on products of the tobacco industry, and, that is to say, on blanks for making wrappers of smoking articles, on webs of wrapping material, on wrappers of smoking articles, on partially ready packets or on packets of the tobacco industry, the method comprising the following steps: - conveying the products to be labelled along a feed path through a labelling unit; - providing a first labelling device and a second labelling device of the labelling unit, which are positioned along the path; - applying to each product a relative label by means of at least one of the two labelling devices (501; 502); - optically checking each labelled product at least at the label and identifying any labelled products in which the label is applied in a non-conforming way compared with a reference labelled product; wherein in a condition of adjusting the labelling unit the method comprises the following steps: - selecting one of the two labelling devices (501; 502) as the main labelling device for applying labels to respective products and selecting the other labelling device (502; 501) as the secondary labelling device; - applying at least one test label to a product by means of the secondary labelling device, obtaining a labelled test product; - adjusting the secondary labelling device if the test label optically checked in the labelled test product is applied in a non-conforming way; characterized in that the method further comprises the steps of: - performing said step of adjusting the secondary labelling device during application of labels by means of the main labelling device; - temporarily pausing the main labelling device, before application of the test label by the secondary labelling device and - activating the main labelling device for applying labels to respective products, after application of the test label and during the step of adjusting the secondary labelling device. - conveying the products to be labelled along a feed path through a labelling unit; - providing a first labelling device and a second labelling device of the labelling unit, which are positioned along the path; - applying to each product a relative label by means of at least one of the two labelling devices (501; 502); - optically checking each labelled product at least at the label and identifying any labelled products in which the label is applied in a non-conforming way compared with a reference labelled product; wherein in a condition of adjusting the labelling unit the method comprises the following steps: - selecting one of the two labelling devices (501; 502) as the main labelling device for applying labels to respective products and selecting the other labelling device (502; 501) as the secondary labelling device; - applying at least one test label to a product by means of the secondary labelling device, obtaining a labelled test product; - adjusting the secondary labelling device if the test label optically checked in the labelled test product is applied in a non-conforming way; characterized in that the method further comprises the steps of: - performing said step of adjusting the secondary labelling device during application of labels by means of the main labelling device; - temporarily pausing the main labelling device, before application of the test label by the secondary labelling device and - activating the main labelling device for applying labels to respective products, after application of the test label and during the step of adjusting the secondary labelling device.