Dr Libby Wilson, a Glasgow-based retired GP and founder member of the Friends at the End (Fate) was questioned after she admitted offering advice and emotional support

to Cari Loder, 48, who had lived with the neurological disease for many years and ended her life at her home in Surrey, on June 8.

Dr Wilson yesterday attended a police station in Woking by appointment and was quizzed by officers. She was arrested on suspicion of aiding and abetting suicide and conspiring to aid and abet suicide, and released on bail to return on November 18.

A man in his 70s from the Godalming area of Surrey, and a man in his 50s from west London, have already been arrested on suspicion of helping Ms Loder end her life. They have been released on bail until November 18.

Under current legislation, those who "aid, abet, counsel or procure" someone else’s death can be prosecuted and jailed for up to 14 years.

New guidelines clarifying when people are likely to be prosecuted were published last Wednesday by the director of public prosecutions, Keir Starmer QC.

He stated that charges should not be brought if the person who dies has a "clear, settled and informed wish to commit suicide", but people who put pressure

on somebody will face prosecution.

Dr Wilson, a family planning expert, who was understood to be with relatives in Surrey yesterday, has spent more than three decades campaigning for the legalisation of euthanasia, and formed the organisation Fate to lobby the government for a change in the law, and to "spread knowledge about end- of-life choices".

Colleague Sheila Duffy of Friends at the End said the arrest was "absurd". She added: "You do wonder whether police do not have better things to do than to take an 83-year-old woman into custody to question her about giving advice about killing yourself.

"You can find all this stuff on the internet. We also have books about it. Dr Wilson has been helping and advising people for some years and there seems to be some speculation that we are deliberately engineering this to highlight the issues which is not the case."

Ms Loder, a former lecturer at London University’s Institute of Education, who believed she had discovered a treatment for MS, wrote a book about her experiences called Standing In The Sunshine.

But her condition eventually deteriorated and she took her own life in June.

Following Ms Loder’s death, Dr Wilson said the 48-year-old was "terrified" of being sent to a care home, and wanted to end her life while she still could.

She said she "wouldn’t be surprised" to face questions over her involvement.

"She (Ms Loder) was in good spirits and a determined mindset," Dr Wilson said. "She was calm, rational, reasoned, articulate and intelligent. She knew exactly what she wanted to do.

"For her, losing her independence was the ultimate loss of dignity. She could not stand the idea. She knew her condition was worsening and she told me she wanted to do this sooner rather than later."

More than 100 Britons have ended their lives at the Dignitas clinic in Switzerland.