UPDATE 3-UCB stock rises as U.S. clears arthritis drug

* Rheumatoid arthritis major new market for Cimzia

* UCB drug competes with established anti-TNF blockbusters

* Shares up 15.6 percent; Nektar rises as much as 30 pct (Adds analyst reaction, latest shares)

By Philip Blenkinsop and Antonia van de Velde

BRUSSELS, May 14 (Reuters) - Belgian pharmaceutical group UCB (UCB.BR) received a major boost on Thursday when U.S. regulators approved its rheumatoid arthritis drug Cimzia, and its stock rose 15.6 percent.

Shares of UCB's U.S. partner Nektar Therapeutics (NKTR.O) also rose as much as 30 percent to $6.59 on Nasdaq, touching their highest point in a year.

The news will reduce UCB's reliance on older drugs, some of which have lost patent protection, and is a vindication of its investment in biotechnology. Cimzia is a biological medicine given by injection.

"It is a massive sigh of relief," said ING analyst Mark Clark. "The timing is a surprise to everyone as the company had talked about having to supply extra information (to U.S. authorities). A decision was only expected in the fourth quarter."

Analysts at Morgan Stanley said the approval was an important milestone that reduced the risk of investing in UCB, adding that the dosing regimen was better than expected.

UCB, which has a collaboration on Cimzia with U.S.-based Nektar, will pay the smaller company royalties on sales.

Nektar also would get manufacturing revenue during research, clinical development and commercialization of the drug.

Cimzia, already cleared to treat bowel disorder Crohn's disease in the United States, would be made available this week, UCB said in a statement.

It may be administered either at two or four weeks after initial doses, as a monotherapy or with the older drug methotrexate.

"Not only does Cimzia underpin 20 percent of our 30 euros fair value, it is a major driver of UCB's deleveraging strategy," Morgan Stanley said in a research note on Thursday.

UCB has a high level of debt compared to its rivals following recent acquisitions, with around 2 billion euros of debt requiring refinancing in 2010/11.

Shares in the company closed up 3.02 euros to 22.40.