FRANKFURT, Jan 25 (Reuters) - Britain's drug regulator on Thursday said it approved Eli Lilly's injection pen to administer Mounjaro to treat obesity and diabetes, making Britain the first European market to green light the drug in the more convenient device.

The four-dose pen, covering a month's treatment, is branded as Kwikpen, the Medicines & Healthcare products Regulatory Agency (MHRA) said in a statement on Thursday.

The drug itself was cleared for weight loss by the British regulator in November, when the U.S. Food and Drug Administration also gave its go ahead for the wider use under the brand name Zepbound. The U.S. drugmaker put the UK product launch on hold while the injector pen was still under regulatory review.

Tirzepatide is the active ingredient in Mounjaro, which will compete with Novo Nordisk's Wegovy (semaglutide) injection drug in a booming weight-loss market.

Lilly previously said the injection device would be different from the pen Lilly uses for the weight-loss drug in the United States.

The company has also said that in Britain the pen would be for both type 2 diabetes and obesity patients. The class of drugs that Mounjaro and Wegovy belong to was originally developed to treat type 2 diabetes but was found to also promote weight loss.

Other European markets - Germany, Switzerland and Poland - have seen Mounjaro launched before Britain. In those countries, the drug has for now been packaged in vials and patients need to draw the medication into syringes before injection.

(Reporting by Ludwig Burger; Editing by David Evans and Bill Berkrot)