It was ruled the court's decision breached Article 7 of the code, which requires "effective public participation in plans or programmes relating to the environment".
The ruling means the government will have to justify its position to the Aarhus Compliance Committee - potentially delaying the project being given Royal Assent next year.
The original Supreme Court action was brought jointly by Hillingdon Council and the Amersham-based HS2 Action Alliance, with the two groups complaining a proper environmental assessment of the controversial multi billion pound scheme should have been carried out before the public were asked for their views on the proposals.
In their submissions ahead of January's court hearing the two groups said: "The consultation had to be 'within a transparent and fair framework, having provided the necessary information to the public' as required by Article 7 [of the Aarhus Convention].
"It failed to do so...the public consultation was not effective because members of the public were not provided with sufficient information for them to make an informed judgment as to the relative environmental and other advantages and disadvantages of HS2 compared to the alternative options. Members of the public were given an incomplete picture of the environmental effects of HS2."
And the Aarhus Committee has ruled in favour of the two groups, and it will now make a decision at its Geneva headquarters within the next six months. It has the power to find that the Supreme Court’s decision leaves the UK in breach of its UN Treaty obligations under the Aarhus Convention.
Hillingdon Council Leader Ray Puddifoot said: "I believe that the UK’s Supreme Court’s decision that a Strategic Environmental Assessment was not required in the HS2 consultation process is inconsistent with laws to protect the environment, leaving the UK in breach of the UN Treaty obligations under the Aarhus Convention. I am therefore delighted that the Aarhus Compliance Committee will be investigating this matter."
Hilary Wharf, director at the HS2 Action Alliance, said: "This decision is a crucial step in getting the government to face up to their environmental responsibilities that they have tried to evade by adopting the Hybrid Bill method of getting planning consent for HS2.
"We have always said that the UK government was wrong but the UK courts have favoured the government. The Aarhus Compliance Committee, part of the United Nations Economic Commission for Europe, has now recognised that our case against the Government needs to be heard."
