1. Work carried out by an employee on fixed or floating installations positioned on or above the part of the continental shelf adjacent to a Contracting State, in the context of the prospecting and/or exploitation of its natural resources, is to be regarded as work carried out in the territory of that State for the purposes of applying Article 5(1) of the Convention of 27 September 1968 on Jurisdiction and the Enforcement of Judgments in Civil and Commercial Matters, as amended by the Convention of 9 October 1978 on the Accession of the Kingdom of Denmark, Ireland and the United Kingdom of Great Britain and Northern Ireland, the Convention of 25 October 1982 on the Accession of the Hellenic Republic and the Convention of 26 May 1989 on the Accession of the Kingdom of Spain and the Portuguese Republic.

2. Article 5(1) of that convention must be interpreted as meaning that where an employee performs the obligations arising under his contract of employment in several Contracting States the place where he habitually works, within the meaning of that provision, is the place where, or from which, taking account of all the circumstances of the case, he in fact performs the essential part of his duties vis-à-vis his employer.

In the case of a contract of employment under which an employee performs for his employer the same activities in more than one Contracting State, it is necessary, in principle, to take account of the whole of the duration of the employment relationship in order to identify the place where the employee habitually works, within the meaning of Article 5(1). 

Failing other criteria, that will be the place where the employee has worked the longest. 

It will only be otherwise if, in light of the facts of the case, the subject-matter of the dispute is more closely connected with a different place of work, which would, in that case, be the relevant place for the purposes of applying Article 5(1) of the convention.

In the event that the criteria laid down by the Court of Justice do not enable the national court to identify the habitual place of work, as referred to in Article 5(1) of the convention, the employee will have the choice of suing his employer either in the courts for the place where the business which engaged him is situated, or in the courts of the Contracting State in whose territory the employer is domiciled. 

3. National law applicable to the main dispute has no bearing on the interpretation of the concept of the place where an employee habitually works, within the meaning of Article 5(1) of the convention, to which the second question relates.