## Facts

- The United Kingdom and France disputed the delimitation of the continental shelf in the western English Channel and Atlantic approaches, including areas affected by the Channel Islands.
- The parties had agreed on parts of an equidistance line in the eastern Channel but remained divided over (1) shelf areas around the Channel Islands and (2) delimitation methodology in the western approaches.
- Both states were parties to the 1958 Geneva Convention on the Continental Shelf; France had issued statements concerning Article 6 (equidistance unless “special circumstances”).
- By a 1975 arbitration agreement, an ad hoc tribunal seated in Geneva was asked to determine the shelf boundary under applicable international law.
- The tribunal emphasized it could delimit only the continental shelf beyond the territorial sea and could not decide territorial sea boundaries, including in narrow waters near the Channel Islands.
- In 1977 the tribunal issued an award delimiting the shelf by a line defined through specified points connected by rhumb lines; in 1978 it issued an interpretive award clarifying technical and interpretive matters without altering the substantive boundary.

## Issues

1. What international law governed delimitation between the parties: Article 6 of the 1958 Convention, customary law requiring an equitable result, or both.
2. What legal effect France’s Article 6 statements had as reservations versus interpretive declarations in their treaty relations.
3. Whether delimitation should follow an unadjusted equidistance method, an equitable principles approach, or an approach combining equidistance with adjustments for special circumstances.
4. How much weight to give geological “natural prolongation” and coastal proximity in delimiting overlapping shelves.
5. Whether the Channel Islands constituted “special circumstances” justifying reduced (partial) effect in constructing the delimitation line.
6. In the interpretive proceedings, whether the request sought permissible interpretation rather than impermissible modification, and how to understand “true equidistance” and “half-effect” lines.

## Decision

- The tribunal held that Article 6 remained applicable between the parties and was to be applied together with customary equitable principles aimed at an equitable result.
- It treated France’s unilateral statements as reservations only to the extent they clearly excluded or modified legal effect; otherwise they were interpretive declarations that did not prevent Article 6 from operating between the parties.
- It rejected both a purely automatic equidistance rule and a free-standing discretionary equity approach detached from the Convention framework.
- It gave limited weight to geological and geomorphologic factors and declined to treat proximity as an independent controlling method, emphasizing geography and equitable considerations.
- It treated islands as potential “special circumstances” and, for the Channel Islands, used adjusted lines giving the islands reduced influence (often described as “half-effect”) to avoid distortion and inequity.
- It used proportionality (coastal lengths versus attributed areas) as a final check on the equitableness of the result, not as a primary method.
- In the 1978 interpretive award, it confirmed its jurisdiction to interpret, clarified the meaning of “true equidistance” and the “half-effect” approach, and upheld the technical method of using rhumb lines on Mercator charts as acceptable practice.

## Legal Principles

- Where both states are parties to the 1958 Continental Shelf Convention, Article 6’s equidistance–special circumstances rule may apply concurrently with customary international law requiring an equitable result.
- “Special circumstances” permit adjustment of an equidistance line when necessary to avoid inequitable outcomes; the party invoking such circumstances bears the burden of showing inequity from an unadjusted line.
- Islands may be treated as special circumstances and may receive reduced effect where full effect would disproportionately distort the boundary relative to mainland coasts.
- “Natural prolongation” supports continental shelf entitlement but generally has limited operational value for delimiting overlapping shelves compared to coastal geography and equitable criteria.
- Proportionality is not an allocation rule; it functions as an ex post check on whether the line produces a grossly disproportionate result.
- In treaty practice, only statements that clearly purport to exclude or modify treaty obligations operate as reservations; interpretive declarations do not necessarily prevent the treaty rule from applying between the parties.
- Interpretation may clarify a prior award’s meaning and technical application without changing the substance of the boundary; modification is not permitted under an interpretation procedure.

## Conclusion

The tribunal drew a continental shelf boundary between the United Kingdom and France by applying Article 6’s equidistance framework as tempered by customary equitable principles, reducing the Channel Islands’ effect as a special circumstance and using proportionality only as a reasonableness check, while later clarifying technical aspects through a separate interpretive award.


Arbitration Between the United Kingdom of Great Britain & Northern Ireland & the French Republic (Delimitation of the Continental Shelf), 18 I.L.M. 397 (Ct. Arb. 1979)