A British GP arrested last week for helping patients travel to Switzerland to commit suicide has said a landmark ruling on assisted suicide may make the situation more difficult for the terminally ill.

Debbie Purdy, who is suffering from multiple sclerosis, last week won the right to have the UK's law on assisted suicide clarified by law lords. As a result, the British director of public prosecutions (DPP) must state exactly when prosecutions would be launched against those who accompany people to commit suicide outside of the country. Purdy wants her husband to be able to accompany her to a clinic in Switzerland when she chooses to die, without fear of arrest.

But Dr Michael Irwin, who has accompanied several terminally ill people to die at the Dignitas clinic in Zurich, said last night that he fears her case will in fact make it more difficult for relatives to travel with patients. Irwin, who is on police bail for helping to fund a patient's death at Dignitas, said a formal explanation of the law "could open a can of worms".

At least 115 Britons have killed themselves at Dignitas, often accompanied by relatives who risk up to 14 years in prison by travelling with them. Last week the law lords told the DPP, Keir Starmer, to clarify how his department decides whether or not to prosecute in assisted suicide cases.

Irwin fears that this clarification will actually make relatives more vulnerable to prosecution. "I've been concerned about the Debbie Purdy case right from the start," he said. "It may make it more difficult for people because she's asking to have the director of public prosecutions to come out publicly and say what friends and family might face on return.

"It all depends on the guidelines the DPP sets out in September, but he's boxed into a corner by the law lords now... You may now have a situation where people have to re-write a will to leave out a spouse so that they can't be prosecuted. It might have been a mistake to promote the Debbie Purdy case: it has drawn people's attention to the whole issue, which is good, but it could open a can of worms."

James Harris at Dignity in Dying, the organisation which has been promoting Purdy's case, said: "I don't share the concern. We think clarification is better than the current muddle where you have a blanket law which covers all assistance to suicide and isn't enforced."

Irwin was arrested on Friday at a pre-arranged meeting at London's Battersea police station, for giving 58-year-old terminal cancer patient Raymond Cutkelvin £1,500 towards the cost of his assisted suicide in 2007. The doctor had called on the police to arrest him a week after they arrested Cutkelvin's partner of 28 years, Alan Cutkelvin Rees , who accompanied him to the Dignitas clinic.