The House of Lords judgment fails to address the question of whether travelling abroad with a loved one is the criminal act of assisted suicide – but it does require the Director of Public Prosecutions (DPP) to publish guidelines on how he makes a decision to prosecute, or not.

So far, more than 100 Britons have travelled to the Swiss assisted suicide clinic Dignitas, but there has yet to be a single prosecution. The offence carries a maximum sentence of 14 years in prison.

The guidelines would most likely build on the DPP's decision not to prosecute the family of the rugby player Daniel James, who went to Dignitas for an assisted death even though he was not terminally ill. They will have to clearly set out the circumstances in which loved ones would not be prosecuted, giving reassurance to thousands who would otherwise have been deterred, and opening the way to decriminalised suicide tourism.

The guidelines are likely to be sympathetic in nature. Lord Brown has said they should reflect that many people who assist a loved one to die "may fairly hope to be, if not commended, at the very least forgiven".

The House of Lords ruled that the right to a private life under Article 8 of the European Convention on Human Rights extended to a protection of personal autonomy and ultimately the right to have choices over the timing and manner of one's own death. This interpretation is in line with how the European Court of Justice has interpreted the right.

The public policy reasons the DPP was hoping would protect him from having to publish guidelines were rejected by their lordships and quite correctly their lordships are demanding precise guidelines guaranteeing legal certainty.

In its legislative capacity, the House of Lords had earlier rejected Lord Falconer's amendment to the Coroners and Justice Bill which would have legalised assisting someone to travel abroad to die, provided that adequate safeguards were met. Unless Parliament acts to bring about a change in the law, assisting someone to travel abroad could become decriminalised, without these safeguards.

Nick Cartwright is an expert in medico-legal decision-making at the University of Reading and is currently writing a commentary on the case of Debbie Purdy for the "Medical Law Review", the UK's authoritative source of reference on health care and the law, which will involve an analysis of judicial reasoning in the High Court, Court of Appeal and House of Lords