UPDATE 1-Europe prepares for shortages of Genzyme drugs

* EU agency prioritises access to Cerezyme and Fabrazyme

* Move follows production problems at U.S. factory

(Adds detail, background)

LONDON, June 25 (Reuters) - Europe's drug watchdog has recommended a priority access system to deal with a shortage of two top-selling drugs for rare diseases from Genzyme Corp (GENZ.O).

The U.S. company was forced to halt production of Cerezyme and Fabrazyme for Gaucher and Fabry disease last week, after detecting a virus at a Boston plant, and it said stocks would not be sufficient to meet global demand.

To ensure stocks last as long as possible until new batches can be produced, the European Medicines Agency said on Thursday it had agreed temporary changes to the way the medicines are prescribed to patients with the rare, inherited enzyme-deficiency disorders.

For Cerezyme, priority will be given to infants, children and adolescents, and adults with active disease progression. Adults without clinical evidence of active disease are recommended to get a reduced dose.

For Fabrazyme, the priority patients will be children, adolescents and adult male patients. Adult female patients, in whom the disease is less severe, may receive a lower dose.

Industry analysts believe Shire (SHP.L) should see increased demand for its drug Replagal, which competes with Fabrazyme in markets outside the United States, as a result of the supply problems at Genzyme. (Reporting by Ben Hirschler; Editing by Dan Lalor)