The present invention relates to a process for the preparation of sulphated glycosaminoglycans derived from N— acetylheparosan which comprises: a) N-deacetylation and N-sulphation of the N-acetylheparosan polysaccharide prepared from natural or recombinant bacterial strain, preferably K5 E. coli, b) enzymatic epimerization with the glucuronyl C5-epimerase enzyme, c) partial O-sulphation followed by a partial O-desulphation, d) partial 6-O-sulphation, e) N-sulphation and an intermediate step of controlled depolimerization characterised by the fact that both O-sulphations (O-sulphation and 6O-sulphation) are partial.Furthermore the invention relates to the products obtained according to the process which show a ratio between the anti-Xa activity and anti-IIa activity equal to or higher than 1 and to compositions comprising said products in combination with suitable and pharmaceutically acceptable excipients and/or diluent.