* Rheumatoid arthritis major new market for Cimzia

* UCB drug competes with established anti-TNF blockbusters

* Shares up 21.59 percent

(Adds analyst reaction, latest shares)

By Philip Blenkinsop and Antonia van de Velde

BRUSSELS, May 14 (Reuters) - Belgian pharmaceutical group UCB (UCB.BR: Quote, Profile, Research) received a major boost on Thursday when U.S. regulators approved its drug Cimzia for the treatment of rheumatoid arthritis, sending the stock 18 percent higher.

The news will reduce the group'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.