In time of war, a merchant navy ship belonging to the enemy could be stopped on the high seas, with or without the use of force. The captor would normally have two options; either to order passengers and crew off the ship and proceed to sink her to deprive the enemy of the cargo, or the captain of the warship could place a small group of his crew on board armed and prepared to subdue the crew. The armed group, a Prize Command, they would come onboard with orders to sail the ship to a port belonging to the conquering side. Such a ship would normally be referred to as a Prize. Even civilian merchant navy vessels belonging to a neutral nation could be taken as a Prize, if they were thought to be serving the ‘other side’ in the conflict, ie if they were thought to be carrying cargo or personnel useful to the war effort, such items would be classed as ‘Contraband’, and the cargo was destined for an enemy port.
The British definition of contraband had from 1801 simply been goods that were “designed for, and contributive to the advantage of the enemy”.On 4 September 1939 a very brief list of what constituted contraband was published in the London Gazette.  As expected, arms, ammunition, chemicals, machines or their components, any materials etc. were on the list classified as ‘absolute contraband’. On the same list was fuel and ‘all contrivances for, or means of, transportation on land in the water or air...” and animals, components etc. to produce any of this. Interestingly Britain also had a category called ‘conditional contraband’ which covered all food, clothes and materials for this.In reality, the conditional contraband category was likely to be more of a diplomatic nicety than a way of avoiding capture.Germany had a similar grading.
Britain had learned during the First World War that it was unrealistic to even attempt to search an entire freighter to check their cargo whilst on the high seas. They instituted contraband control points where they sent ships with a Prize Command onboard to be checked thoroughly. Germany, on the other hand appears to have gone by the ship’s papers, supported by an inspection to decide, they simply did not have any suitable ports outside the North Sea.
In his unfinished 2 volume history of the British Navy, Sir Nicholas Harris Nicolas (1799-1848) argued that ‘Prize-money seems to have been as ancient as the English Navy itself’. It was certainly a very convenient way of encouraging the officers and crew to ‘show commitment and determination’ to an attack on an enemy ship, or one carrying forbidden cargoes to the enemy. It was also a way of legitimising privateers. By granting them permits, they would become legal seafarers. Alfonse III, the King of Aragon did so in 1228. If such a ship was caught by a belligerent and brought to one of his ports, a national Prize Court would determine whether the catch could be declared ‘a good Prize’ or not. If the court’s decision was in favour of the captain of the occupying ship, the prize would be sold at auction and the proceeds shared in such a way that officers and crew all benefited. The crew of the Prize would be sent home if the ship was from a neutral nation, or imprisoned if it belonged to the enemy. It would only be a ‘good Prize’ if the ship was captured in international waters, or in the waters of the belligerent. If such a capture took place in neutral waters, the Prize Court would normally be expected to restore the ship to its captain and crew.
The Bergen U-Boat bunker
The practice of taking a Prize was a common part of warfare up to and including the 20th Century, which was why this was planned to be dealt with in a Hague Convention (1907) all of its own. It was probably also the reason why this Convention was never ratified; the practice had proven to be too lucrative and had too many interested parties and interpretations. This failed convention would have provided a higher, international court of appeal for national Prize Court decisions. When the 1907 proposal failed, international lawyers continued to work on this and a new proposal was introduced to the 1908 London Naval Conference. This had been brought together solely for the participating nation states to agree a set of rules for naval warfare. Again, the proposal failed to gain the prerequisite level of international support and the London Declaration of 1909, never became law, thereby sealing the fate of the International Prize Court for ever, although it has been argued that several belligerents were abiding by the agreed naval war conduct during WWI.  Britain was one of the countries who did not ratify the Declaration. As such, the still not ratified Prize Law was included in the authoritative Oxford manual on the Law of Naval War of 1913. It carried no authority, no international approval, but it was still an issue to be considered in war at sea. In fact, the only agreement that dealt with issues around privateering and contraband was the 1856 Paris Declaration Respecting Maritime law. This has basically remained in force until subsumed into the San Remo Declaration of 1995.
Even into the 21st Century Prize Law still prevails. The San Remo Manual on International Law Applicable to Armed Conflicts at Sea includes prize law. Even yachts may still be captured. The San Remo Manual appears to recognise modern day applicability based on some very recent cases.
Admittedly, its application since WWII has been very limited, but very significant. It was quite salient in the Israeli – Arab conflicts and during the India-Pakistan conflict of 1965 and more recently during the 1990 Iraq war, when the Coalition Maritime Interception Force controlled the Persian Gulf to ensure that only goods approved by the UN Security Councilresolutions. All banned good, known as contraband, would be stopped from being imported to Iraq or from being exported.
The British Joint Service Manual of the Law of Armed Conflict of 2004 includes a very detailed section on the capture of civilian enemy ships as prize. The British view, departs from the San Remo Manual, however, in the use of Prize Courts. Britain has apparently not used such courts for some time.If British forces capture a ship, it is regarded as British property according to the manual, of that, there is no doubt. No court ruling is required to say so.
When the German inspection team checked the papers and the cargo manifest for the City of Flint, they soon discovered that the cargo, albeit in transit, bound from a neutral port to an enemy one, contained items classified as contraband under the German Prize Regulations of 28 August 1939. It is hard to imagine how this set of rules could be avoided in a 4000t cargo. Not only straight forward goods of a military nature were banned, as were all information, maps and related instruments; photographs and related instruments and equipment or anything relevant to the production of such articles.... food, including live animals, shoes or clothing or materials to be used in the production of such. Not all the cargo was bound for British ports, some was heading for Ireland. Nevertheless, the fact that an enemy nation would benefit from some of the cargo, was enough to support the German case of declaring the City of Flint a Prize, placing a Prize Crew on board and ordering the ship to sail to a German port. It was all done in accordance with the prize regulations, and differs little from what a similar incident would set in motion today.
At the end of August 1939, the German fruit carried Pomona, of the Afrikanische Frucht Compagnie, was stuck in the South West India Dock in London with engine trouble. The ship was only a year old, and when war seemed imminent, British authorities slowed down the delivery of the relevant spare parts. At the outbreak of war on 3 September, the Ministry of War seized the ship as a Prize. She was renamed British Empire Merchant. Interestingly, the British Prize Court in London only sat for the first time on 2 November 1939.  This did not stop the Prime Minister at the end of September 1939, when speaking in Parliament, to reel off a list of around 338,000 tons of contraband intercepted and stopped from reaching Germany. The contraband control system of checking all merchant shipping moving in to the North Sea was proving to be effective.
 Thompson, p. 140
 Supplement to the London Gazette, 4 September 1939
 Nicolas, volume 1 page 140, as quoted in Clowes, p.117
 Hongsheng, S. The Evolution of Law of War; Chinese Journal of International Politics, Vol. 1, 2006, pp267-301
 Kraska, James; Prize Law, (July 1, 2011) Max Planck Encyclopaedia of Public International Law, Forthcoming
 Ministry of Defence (2004): The Joint Service Manual of the Law of Armed Conflict, JSP 383
 Seekriegesrechtliches Sammelheft, Heft1 Prisenordnung, Berlin 1940 pp. 20-21
 Jordan, Roger W.: The World’s Merchant Fleets, 1939 p. 476 and correspondence with author
 Kunz, Josef L. : British Prize Cases 1939-1941. The American Journal of International Law. Vol. 36, No. 2 (Apr. 1942) pp.204-228