The opinion (pdf 26 pages) is by Sir David Edwards KCMG PC QC, Sir Francis Jacobs KCMG PC QC, Sir Jeremy Lever KCMG QC (Retired), Helen Mountfield QC and Gerry Facenna QC. Bindmans refer to it as The Three Knights Opinion.
The important question of whether an Article 50 notification may be unilaterally withdrawn was not answered by the Brexit litigation which proceeded on the basis that it could neither be withdrawn unilaterally nor given conditionally. (Please see this previous post - Absent from the Feast). The government's position was, and as far as I know remains, that once notice is given it would never be withdrawn. Even accepting that negotiations are not going to result in an agreement that is as good as the UK's full membership, the government's stance could lead to problems if negotiations fail to reach any agreement or perhaps reach an unsatisfactory agreement.
The Three Knights Opinion states (para 11) that - "Contrary to the common ground in Miller, ....., we believe the better view to be that a notification under Article 50(2) can be given in qualified or conditional terms and can be unilaterally withdrawn."
The Three Knights argue that a clause should be inserted into the European Union (Notification of Withdrawal) Bill 2017 to make it clear that the UK shall leave the EU when Parliament has either legislated to approve the terms of a withdrawal agreement or has legislated to authorise withdrawal without agreement. If Parliament were to refuse to give such approval, the notification would then either lapse or could be withdrawn unilaterally Article 50 cannot have the effect of ejecting a Member State from the EU contrary to its own constitutional requirements.
There can be no certainty about what the outcome of negotiations will be and it is almost impossible to accurately forecast what world conditions will be in 2 years time. The insertion of the suggested clause therefore seems advisable.
Nevertheless, as noted previously, Article 50 sets out two sequential steps: Decision Art 50(1) and the Notification 50(2). The Decision has to be made in accordance with constitutional requirements. Thus, it seems entirely logical to say that notification ought not to be given until the constitutional requirements for deciding to leave have been complied with and the EU ought to be entitled to rely on that.
The Three Knights have made a valuable contribution but it remains an opinion albeit an eminent one. What beats me is why doesn't somebody actually find out what the correct position is! A privately funded attempt to do so is in progress via the courts in the Republic of Ireland.