BRUSSELS/MADRID (Reuters) - Picking Amsterdam, Barcelona or Vienna as the new headquarters of Europe’s drugs regulator after Brexit would be the best option for retaining staff, according to a survey of its workers.

The European Medicines Agency (EMA) on Tuesday warned that it could lose more than 70 percent of its staff, making it unable to function, if politicians pick an unpopular base for the London-based agency once Britain leaves the European Union.

It cited a survey of its around 900 staff, but has declined to rank the 19 candidate cities.

The survey results, reviewed by Reuters, put Milan and Copenhagen in fourth and fifth place, respectively after the three most popular.

Dutch officials, who asked not to be named, said they understood they were first choice among the EMA’s workers, hoping this would be taken into account by the European Commission when it made its recommendations.

The Commission is assessing new locations, but the decision rests with EU leaders who will try to reach a deal at their next summit in October, with a final decision expected a month later.

The EMA, which declined to comment, sees keeping staff as key to maintaining essential services such as new drug approval and monitoring side effects, following the planned move from London.

In its statement on Tuesday, the watchdog warned that only the five most popular of the 19 candidate cities would permit it to largely maintain key functions. Even then it would take to three years to fully recover from the disruption.

The EU’s need to ensure business continuity could clash with another EU ambition -- sprading the bloc’s agencies more evenly across Europe and giving newer, eastern member states a chance to catch up.

All of the top five candidate cities favored by EMA staff are in countries that already host one or more EU agencies. Some of the lower ranking cities are in Bulgaria, Croatia, Slovakia and Romania, where there are none.

Athens, Dublin and Stockholm are also among the aspiring host cities, while Milan had previously been seen as a possible frontrunner.