The “Rule Of Law” means that all in society, are subject to the law. Including the government. There must be a lawful basis for what the government does or denies to do. Including the international treaty obligation, abiding by commitments is part of the rule of law. Above mention points are regardless of whether domestic law needs to be made or changed to implement a treaty.
If an act were passed into statute, laying it before the parliament would breach the treaty obligation. It would amend domestic legislation.
A bill stands as an Act Of Parliament only after being passed by the legislature. After the bill becomes an act, the government is required to proceed in accordance with the Rule Of Law. If the legislature passed such a bill and it became an Act of Parliament, the Rule of Law requires the government to proceed following it. That is what parliamentary sovereignty, or to be more precise the sovereignty of the Crown in Parliament, means. Whether passing such an Act of Parliament gives rise to a claim under the treaty (to which the domestic legislation is unlikely to be any defence) is a separate issue. But again, there is no breach of the rule of law.
An alternative proposition is that the rule of law precludes a government from even laying a bill before parliament which, if passed, would put the UK in breach of a treaty obligation? Or is it to be said that the rule of law requires that such an Act of Parliament should itself be deemed by our courts to be unlawful or of no consequence?
No legal basis for any such proposition is visible.
A bill and resultant Act of Parliament might be unwise or foolish or damaging to the UK’s interests. As a matter of fact, those are not legal questions. It can not make any moral difference to the analysis. To take two points which have been made repeatedly over the past few days – the treaty in question was signed recently. All carried out by the same government.
Contracts should be honoured, and treaties should be kept. The (different) principle of pacta sunt servanda suggests. A breach of contract does not itself entail a violation of the rule of law. Breaching a treaty obligation because parliament has so legislated does not either.
I do not consider that there is a breach of the rule of law in the government’s approach. Whether it is wise or unwise is a different question – a political and not a legal one – and not the subject of this argument.