LONDON (Reuters) - U.S. drugmaker Pfizer said on Wednesday it had won approval from the European Union for its best-selling breast cancer drug Ibrance, making it the first medicine of its kind to reach the European market.

The decision, which had been expected following a positive recommendation from the European Medicines Agency in September, comes as Novartis readies a rival treatment called ribociclib.

Both drugs block enzymes known as cyclin-dependent kinases 4 and 6. They are designed be used in combination with existing drugs in women whose tumors grow in response to oestrogen and whose cancer is not caused by the HER2 protein.

Sales of Ibrance, which was approved by the U.S. Food and Drug Administration in February 2015, are forecast by analysts to be some $2.1 billion in 2016, according to Thomson Reuters data.

Novartis plans to file its rival medicine for approval worldwide this year, following promising clinical trial results.

(Reporting by Ben Hirschler; Editing by Greg Mahlich)