The invention relates to a method and unit for forming tubular lengths of web material, and a labelling machine. The method comprises the steps of a) winding a portion (2) of web material in a tubular configuration such that opposite vertical ends (34) of said linear portion (2) overlap; and b) welding said overlapping ends (2) to fix said linear portion (2) of web material in sleeve configuration; characterised in that said step b) comprises simultaneously c) heating up at a first temperature (T1) a first region (S') of said portion (2) over the whole of which said ends (34) overlap; and d) heating up at a second temperature (T2) lower than said first temperature (T1) a second region (S'') of said portion (2) adjacent to said first region (S'), said ends (34) of said portion (2) of labelling web material overlapping over only a part of said second region (S''). a forming unit (15) configured to implement the method outlined above.