'Royal Marine' was the British mining offensive against German riverine and canal traffic (10/31 May 1940).

The plan was to destroy German bridges, barges and other water transport. After several postponements insisted on by the French government, fearful of German retaliation, the operation began on 10 May 1940, when the German offensive in the west began.

The mines caused some damage and delay to German river traffic on the Rhine from Karlsruhe to Koblenz and damaged bridges and protective barriers. Part of the plan was for bombers of RAF Bomber Command to drop the mines into rivers and canals on moonlit nights. but this had hardly begun when the campaign ended. The success of the plot was nullified by the Allied defeat and the Franco-German armistice of 22 June.

Despite the concerns of the French government during the 'Phoney War' in case the Germans responded to the Allied mining campaign with air attacks and reprisals against French waterways, it was intended that 'Royal Marine' would take place simultaneously with the 'Wilfred' undertaking to mine Norway’s coastal waters. The novelty of 'Royal Marine' was intended to divert US attention from the possible illegality of 'Wilfred', which was intended to force German convoys transporting Swedish iron ore from the Norwegian port of Narvik out of the relatively invulnerable waters of the Leads between the Norwegian mainland and the strings of close offshore islands out the into international waters, where they could be attacked by British warships.

Simultaneous attacks with river mines against Germany was intended to deflect criticism that the Allies were not making war on Germany but the small countries around it which the UK and France claimed to be protecting. A decision of the Anglo-French Supreme War Council was taken on 28 March to start 'Royal Marine' on 4 April and the air-dropping of mines on 15 April. The decision was vetoed shortly afterward by the French War Committee, a ruling which was not rescinded for about three months. 'Wilfred' was left to take place on its own on 5 April and was then postponed to 8 April, later parts of the plan being cancelled when news arrived that the German fleet had sailed for what became the invasion of Norway. The British and French were able to agree that 'Royal Marine' would begin as soon as the Germans started their offensive in the west.

The core plan had been presented to the British cabinet in November 1939 by Churchill, as a means of retaliation against illegal German minelaying. (Brigadier E. L. Spears claimed that he had originally suggested the idea to Churchill as they visited eastern France in August 1939, but by the time the operation began, Churchill believed the idea to be his own.) A stock of 2,000 riverine mines, with more in production at the rate of 1,000 per week, was to be put into French rivers which flowed into western Germany, by naval parties led by Commander G. R. S. Wellby. The sailors were to be based in the 'Ligne Maginot', about 5 miles (8 km) from the Rhine river, to deposit this river, interfering with commercial traffic for 100 miles (160 km) downstream of Karlsruhe.

The mines would affect barge traffic and other river craft, but become inert before reaching neutral territory at the border of Germany and the Netherlands.

On 6 March the cabinet was notified that mines would be ready for release from river banks on 12 March and to be dropped by RAF bombers by the middle of April, between Bingen am Rhein and Koblenz on moonlit nights. Neutrals were to be warned. The first 300 to 400 riverine mines were ready by the night of 14/15 March, but after French objections for fear of German retaliation, the plan was postponed. During April, Churchill tried to persuade the French to drop their objections to 'Royal Marine' and remarked after meeting the French prime minister, Édouard Daladier, 'Nous allons perdre l’omnibus' (We will miss the bus'.

The mines had been specially developed by Ministry of Defence 1 (MD1, the so-called 'Churchill’s toyshop'), a British weapon research and development organisation. The mine, known as the 'W' Bomb, was designed by Millis Jefferis, who had received the request for the device on 10 November and had completed the first demonstration model by 24 November. A delayed-action fuse based on a soluble chemical pellet was devised by Jefferis’s assistant, Stuart Macrae, using an Alka-Seltzer tablet, which had been found to dissolve at a predictable rate. Each mine contained 15 lb (6.8 kg) of TNT. Trials were carried out in the Thames river during in December 1939, and these revealed that, depending upon type, the mines either floated or bounced along the river bed. Because Jefferis’s department comprised only three people at this time, the trials had to be conducted with the help of a boat crewed by local Sea Scouts, who followed the mines after they had been dropped from Chiswick bridge. More than 20,000 'W' Bombs were produced in the course of the war.

On 10 May, the first mines were released into the Moselle river to destroy pontoon bridges built by German army engineers; other mines were put into the Rhine river to negligible effect. On 13 May, the British put 1,700 mines in the Rhine river near Soufflenheim, and these were reported by Gėnėral Victor Bourret, commander of the French 5ème Armėe, to have caused damage to the barge barrier protecting the bridge at Karlsruhe. Several pontoon bridges were also damaged and river traffic was temporarily suspended between Karlsruhe and Mainz. By 24 May, more than 2,300 mines had been released into the Rhine, Moselle and Meuse rivers. On 9 June, Gėnėral de Armée Andrė Gaston Pretelat, commander of 2ème Groupe d’Armėes, ordered the mines to be sent down the river Rhine in order to delay a German attack on the 'Ligne Maginot'. The air-dropping of mines by RAF Bomber Command began into the Rhine river between Bingen and Koblenz, and other drops were made into canals and river estuaries debouching into the Heligoland Bight, but only comparatively small numbers of mines were laid by aircraft before the end of the Battle of France ended, and the damage caused could not be measured.