South Korea's Samsung Bioepis and its partner Biogen have launched a third rival to AbbVie's blockbuster medicine Humira in Europe, ramping up competition for a slice of the world's top-selling prescription drug's sales.

Wednesday's launch follows similar moves a day earlier by Amgen and Novartis's Sandoz unit. A fourth so-called biosimilar copy of Humira is expected soon from Mylan.

The primary European patent on Humira expired on Oct. 16, opening the door to cheaper near-identical copies for treating rheumatoid arthritis, Crohn's disease, ulcerative colitis and psoriasis.

Humira's worldwide sales were $18 billion last year, of which around $4 billion came from Europe, and healthcare providers are hoping for significant savings with the arrival of cut-price rivals.

Samsung Bioepis is a joint venture between Samsung BioLogics and Biogen.

News of the launch comes amid a slide in Samsung BioLogics shares after a local media report that said the company faces disciplinary action due to alleged accounting violations.

(Reporting by Ben Hirschler; editing by Jason Neely) ((ben.hirschler@thomsonreuters.com; +44 7542 5082; Reuters Messaging: ben.hirschler.thomsonreuters.com@reuters.net))