Question: In July 2019, the [United Kingdom seized an oil tanker carrying oil from Iran to Syria](https://www.theguardian.com/world/2019/jul/04/royal-marines-gibraltar-tanker-oil-syria-eu-sanctions). Reportedly, this is to enforce EU sanctions against Syria. But neither Iran nor Syria are members of the EU, so how can EU sanctions against Syria be of any relevance? Do sanctions imply a [blockade](https://en.wikipedia.org/wiki/Blockade)? Do EU sanctions authorise EU member states to check and possibly impede any vessel transiting through EU waters, or is such passage supposed to be free under [innocent passage](https://en.wikipedia.org/wiki/Innocent_passage), [strait passage](https://en.wikipedia.org/wiki/Strait_passage), or [transit passage](https://en.wikipedia.org/wiki/Transit_passage)?

(It gets more complicated, because it was in waters claimed by Gibraltar, a claim that Spain disputes, and the ship was apparently registered in Panama; but I'm not sure those complications affect the answer in this case)

Answer: 
The key legal point of their justification seems to be this, from the [press release](https://www.gibraltar.gov.gi/press-releases/grace-1-confirmed-to-be-carrying-full-load-of-crude-oil-5222019-5112) linked and quoted by Fizz:

> 
> The Grace 1 was detained last week in Gibraltar when it freely
>  navigated into British Gibraltar Territorial Waters to a point two
>  miles off the Eastside of Gibraltar, having previously exited the
>  international waters of the Straits of Gibraltar, on a pre-arranged
>  call for provisions and spare parts.
> 
> 
> 

Article 18 of [UNCLOS](https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf) says this:

> 
> 1. Passage means navigation through the territorial sea for the purpose of:
> 
> 
> (a) traversing that sea without entering internal waters
>  or calling at a roadstead or port facility outside internal waters; or
> 
> 
> (b) proceeding to or from internal waters or a call at such roadstead
>  or port facility.
> 2. Passage shall be continuous and expeditious. However, passage includes stopping and anchoring, but only in so far as the same are
>  incidental to ordinary navigation or are rendered necessary by
>  force majeure or distress or for the purpose of rendering assistance
>  to persons, ships or aircraft in danger or distress.
> 
> 
> 

Once you stop to take on supplies, it's no longer passage.

I also don't think stopping to take on supplies in itself counts as calling at a [roadstead](https://en.wikipedia.org/wiki/Roadstead) or port facility. But even if it did, "passage" only seems to apply to going to or from the place, not actually being at it. Once you've chosen to stop in a country's internal waters or facility, you're placing yourself fully within its jurisdiction.
As per UNCLOS Part 3 Article 38, the right of unimpeded transit passage applies to straits such as Gibraltar and Hormuz. Stopping to take on supplies (if true) could not have altered the vessel’s right to transit passage as it still remained incidental to her ordinary navigation. 

In fact the vessel was still exercising its right of transit passage without any intention of harbouring in Gibraltar before being impeded. 

[![UNCLOS Part 3 Article 38](https://i.stack.imgur.com/efR4l.jpg)](https://i.stack.imgur.com/efR4l.jpg)

> 
> ### Article 38
> 
> 
> *Right of transit passage*
> 1. In straits referred to in article 37, all ships and aircraft enjoy the right of transit passage, which shall not be impeded; except that, if the strait is formed by an island of a State bordering the strait and its mainland, transit passage shall not apply if there exists seaward of the island a route through the seas or through an exclusive economic zone of similar convenience with respect to navigational and hydrographical characteristics.
> 2. Transit passage means the exercise in accordance with this Part of the freedom of navigation and overflight solely for the purpose of continuous and expeditious transit of the strait between one part of the high seas or an exclusive economic zone and another part of the high seas or an exclusive economic zone. However, the requirement of continuous and expeditious transit does not preclude passage through the strait for the purpose of entering, leaving or returning from a State bordering the strait, subject to the conditions of entry to that State.
> 3. Any activity which is not an exercise of the right of transit passage through a strait remains subject to the other applicable provisions of this Convention.
> 
> 
>