Robert and Jennifer Stokes were not terminally ill

The Public Prosecutor's Office in the city of Zurich says it has been concerned for some time about a Swiss voluntary euthanasia charity, Dignitas.

The charity helped a British husband and wife, Robert and Jennifer, Stokes end their lives two months ago even though neither was thought to be terminally ill.

The prosecutor's office said it was particularly worried about the speed with which the assisted suicides took place - in many cases patients were dead within a day of arriving in Zurich.

Founded in 1998, Dignitas has helped more than 100 people die - at least two-thirds of them foreigners.

Under a Swiss law dating back to 1937, assisted suicide is a crime only if those providing assistance can be shown to have acted out of self-interest.

The case of Stokes caused an outcry in the UK when they travelled to Zurich at the end of March.

The Swiss authorities are also looking into the suicide of an elderly German woman who married a much younger man shortly before her death.

The German authorities are already investigating this case.