The acquisition, which said was part of its long-term international expansion plan, will allow the company to provide postal collection and sorting before "final mile" delivery by the Royal Mail.
's customers including Harrods, IPC Media, The Economist, Guardian Media Group and Laura Ashley. It was founded in 1969 and has 200 staff at four sites, with a turnover of £24m a year.
La Poste's UK subsidiary already offers services such as data management, document preparation, mail collection and franking and business reply.
Raymond Redding, managing director of La Poste Group, said: "Now we will be able to operate in the part of the UK mail process which precedes 'final mile' delivery, giving us a deeper understanding of the UK postal market, developing our offering to customers and, in turn, enabling us to increase market share."