The HS2 Action Alliance, Buckinghamshire County Council, Hillingdon London Borough Council, and Heathrow Hub Ltd claimed that the Secretary of State should have done a strategic environmental assessment under Directive 2001/42 before the government's "Next Steps" Command Paper on HS2.

They also argued that a hybrid bill procedure did not comply with the Environmental Impact Assessment Directive 2011/92/EU because the party whipped the vote, and limited opportunity to examine the information in Parliament.

[2] Lord Reed observed that the scrutiny of the legislative process required by the EU directive may amount to an impingement "upon long-established constitutional principles governing the relationship between Parliament and the courts"[3] including the 207.

[3] That conflict between constitutional principles of the EU and UK be decided by courts under the constitutional law of the United Kingdom, rendered EU law’s position in the UK determinant not only by the European Communities Act 1972 but also by a number of other constitutional instruments that recognized fundamental principles “of which Parliament when it enacted the European Communities Act 1972 did not either contemplate or authorise the abrogation.

[3] The judgment also criticized the European Court of Justice’s previous case law on the SEA Directive and EIA Directive where the court interpreted the meaning of certain articles beyond what the European Parliament had prescribed.