* Switzerland has liberal euthanasia laws
* Germans, Britons make up two-thirds of cases
* Neurological diseases most common reason for ending life
ZURICH, Aug 21 (Reuters) - The number of foreigners travelling to Switzerland to commit assisted suicide doubled over a four-year period, a study published in the Journal of Medical Ethics said on Thursday.
In 2012, 172 foreigners took their lives in Switzerland, which has liberal euthanasia rules, up from 86 in 2009, with citizens from Germany and Britain making up almost two-thirds of the total, the study found.
Assisted suicide has been legal in Switzerland since the 1940s, if performed by someone with no direct interest in the death.
"Mercy killing" is also legal in the Netherlands, Luxembourg, Belgium, and some U.S. states but remains illegal in many countries, pushing some terminally ill people in those coutries to travel abroad where they can be helped to die without fear of their loved ones, or doctors, being prosecuted.
Courts in Britain, France and the European Court of Human Rights have been struggling with the delicate issue in recent months.
Neurological conditions, such as paralysis, motor neurone disease, Parkinson's and multiple sclerosis were the decisive factor in almost half of the cases examined in the study.
A rise in the number of foreign assisted suicides has provoked heated debate in Switzerland. In 2011, voters in the canton of Zurich rejected proposed bans on assisted suicide and "suicide tourism". A year later, the national parliament voted against tightening controls on the practice.
An analysis of the 611 cases between 2008 and 2012 found people from 31 countries were helped to die in Switzerland during the period. The median age was 69.
Nearly half came from Germany, while 20 percent were British. Other countries in the top 10 included France and Italy, which both saw particularly steep rises. (Reporting by Caroline Copley; Editing by Robin Pomeroy)