Any reform of the law on assisting suicide is a matter for Parliament, not prosecutors, the Director of Public Prosecutions has told The Times.

Sir Ken Macdonald, QC, said that it would “undermine the rule of law” if he were to say that those who helped loved ones to die could escape prosecution.

He said that the public would be wrong to expect reform of the law in what he called “a very sensitive area” from the Crown Prosecution Service.

“There is a sense in which people are looking to prosecutors to change the law, to solve this problem,” said Sir Ken, 55, who steps down from his post this week. “They want us to impose a policy that would change the law.

“We can’t do that. It would be unlawful and would undermine the rule of law. If the law is going to be changed, it has to be changed by Parliament.”

He was careful not to express a view as to whether the law should be changed, but is widely believed to favour a review. Although suicide is not a crime, aiding and abetting suicide is a criminal offence, punishable by up to 14 years in prison.

Judgment is due this week in a test case on whether he should issue guidance to the CPS on the prosecution of those who help others to commit suicide.

The case has been brought by Debbie Purdy, who has multiple sclerosis. She would like to know the circumstances in which her husband, the Cuban musician Omar Puente, would be prosecuted if he were to help her travel to Dignitas, the Swiss clinic, to end her life.

A file will also be passed to prosecutors, however, seeking a decision on whether the parents of Daniel James, a rugby player who was paralysed from the chest down when a scrum collapsed on top of him, should face criminal charges.

Yesterday, Sir Ken said that the law was clear: assisting suicide is an offence. Prosecutors applied a two-stage test - whether there was enough evidence to press charges and whether doing so would be in the public interest.

“These are very difficult and sensitive cases, a very sensitive area,” he said.Re-cently, the subject had come before the Lords, who voted against a change.

“Prosecution decisions in cases like these are obviously particularly difficult. We are talking about people who normally in a million years wouldn’t think of committing a criminal offence, about decent people under the most enormous pressure when facing bereavement.” Senior prosecutors making the decisions, he said, “become as anxious about these cases as they do about any they consider. On the face of it, the law is very plain. But we are confronting circumstances of great tragedy.”

Each case was looked at on its merits and the code for Crown prosecutors applied, he said. The final decision either way would be approved by him, “but we can’t change the law”, he said.

The CPS had not “prosecuted anyone who accompanied a relative to the Dignitas clinic”, he said, but it also had to be sensitive to the position of vulnerable people who were in need of protection.

“The danger always is that vulnerable people are encouraged to do something they would otherwise not do. So there are arguments on both sides.”

Sir Ken also welcomed the Government’s decision not to pursue measures allowing terror suspects to be held for up to 42 days before trial. There had been a “change of tone” on the matter, he said.

He has been highly critical of the measure, insisting that it was unnecessary because the present law was working and no suspect had fallen through the net because of limits on detention.

He said: “Our constitution is working, our liberal institutions are intact, parliamentary democracy is very vibrant and this was a national debate.

“Parliament considered the issues and came to a conclusion, and I think that’s all very helpful. Obviously, I think they came to the right conclusion.”

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