Operation Pluto (Pipeline Under the Ocean or Pipeline Underwater Transportation of Oil, also written Operation PLUTO) was an operation by British engineers, oil companies and the British Armed Forces to build oil pipelines under the English Channel to support Operation Overlord, the Allied invasion of Normandy during the Second World War.

The British War Office estimated that petrol, oil, and lubricants would account for more than 60 per cent of the weight of supplies required by the expeditionary forces. Pipelines would reduce the need for coastal tankers, which could be hindered by bad weather, were subject to air attack, and needed to be offloaded into vulnerable storage tanks ashore. A new kind of pipeline was required that could be rapidly deployed. Two types were developed, named "Hais" and "Hamel" after their inventors. Two pipeline systems were laid, each connected by camouflaged pumping stations to the Avonmouth-Thames pipeline.

The first was the not-very-successful "Bambi" project, which connected Shanklin on the Isle of Wight to Cherbourg in Normandy. Deployment of Bambi began on 12 August 1944, and it delivered just 3,300 long tons (3,400 t) between 22 September, when the first pipeline became operational, and 4 October, when it was terminated. More successful was "Dumbo", which ran from Dungeness on the Kent coast to Boulogne in Pas-de-Calais. The Dumbo system began pumping on 26 October, expanded to 17 pipelines by December, and remained in action until 7 August 1945. Ultimately, the pipelines carried about 8 per cent of all petroleum products sent from the United Kingdom to the Allied Expeditionary Force in North West Europe, including some 180 million imperial gallons (820 million litres) of petrol.

In early April 1942, the Chief of Combined Operations, (then Vice-Admiral, later First Sea Lord) Lord Louis Mountbatten, approached the Secretary for Petroleum, Geoffrey Lloyd, and asked if an oil pipeline could be laid across the English Channel. Mountbatten was tasked with planning the Allied invasion of German-occupied Europe, and had concerns about the supply of petroleum products, since it was considered unlikely that a port with oil reception facilities could be quickly secured. The British War Office estimated that 60 per cent or more by weight of the supplies of the expeditionary forces would consist of petrol, oil and lubricants (POL). In the initial stages of the assault, packaged fuel would be supplied in 20-litre (4.4-imperial-gallon) jerricans and 44-imperial-gallon (200-litre) drums. To supply the twenty million jerricans required, an entire American manufacturing plant was shipped to the London area, where it was operated by the Magnatex firm under the supervision of the Ministry of Supply. By 1944, a stockpile of 250,000 long tons (250,000 t) of packaged petrol and diesel fuel had been accumulated in the UK.

After the first few days of the invasion, it was hoped that petroleum could be supplied in bulk. Pipelines were not the sole or even the principal means by which Combined Operations was contemplating supplying bulk petroleum; it intended to rely primarily on small shallow-draught coastal tankers, of which thirty were under construction. American 600-deadweight-ton (610-deadweight-tonne) "Y" tankers began arriving in the UK in the spring of 1944. In 1943, the British also initiated a programme to construct 400-deadweight-ton (410-deadweight-tonne) Channel tankers (Chants), but only 37 were completed by May 1944. It was hoped that petroleum products might also be supplied by ocean-going T2 tankers lying offshore through ship-to-shore pipelines. The project to develop these pipelines was codenamed Operation Tombola, and the pipelines themselves became known as Tombolas. The submarine pipeline had sufficient advantages to make it worthwhile to explore as a backup means of supply. Submarine pipelines were less susceptible to enemy air attack and the frequently stormy English Channel weather, and their use would reduce the forces' dependency on vulnerable storage tanks ashore.

Lloyd consulted his expert advisors: Brigadier Sir Donald Banks, the director-general of the Petroleum Warfare Department; Sir Arthur Charles Hearn [de], a former director of the Anglo-Iranian Oil Company and the oil advisor to the Fourth Sea Lord; and George Martin Lees, an eminent geologist. At the time, submarine pipelines were in use in ports and over short distances, but no pipeline had ever been laid across such a great distance or under the currents and tidal conditions found in the English Channel. Moreover, to minimise interference by the enemy and the effect of the tides, the entire pipeline would have to be laid in a single night. They regarded the proposal as infeasible using any known method of construction of pipelines 6 inches (15 cm) or more in diameter.

The Chief Engineer of Anglo-Iranian, Clifford Hartley, was visiting the Petroleum Warfare Department at this time, and he heard about the proposal, and was convinced that it was possible. In the hilly terrain of Iran, Anglo-Iranian had employed a 3-inch (7.6 cm) pipeline. Running at 1,500 psi (10,000 kPa), it delivered 100,000 imperial gallons (450,000 L) per day, the equivalent of over 20,000 jerricans. On 15 April he pitched his proposal for a continuous length of pipeline similar to a submarine communications cable without the core and insulation, but with armour to withstand the internal pressure, which could be deployed by a cable-layer ship. Additional capacity could be obtained by laying multiple lines. By using high pressure, the line could carry different kinds of fuel. At low pressure different fuels would mix, but at high pressure they would stay separate. Thus, the pipeline could be used for aviation spirit, and then switched to diesel fuel.

The project was given the codename Pluto, which stood for "pipeline underwater transportation of oil" or "pipeline under the ocean". The operation was placed under the chief of staff to the Supreme Allied Commander, Designate (COSSAC). The G-4 section of the COSSAC staff, which assumed responsibility for Pluto, was headed by British Major General Nevil Brownjohn, with American Colonel F. L. Rash, Colonel Frank M. Albrecht, and Major General Robert W. Crawford successively as his deputy. Royal Navy Captain John Fenwick Hutchings from the Admiralty's Department of Miscellaneous Weapons Development was placed in command of Operation Pluto. By VE-Day his command would consist of several ships, over 100 merchant navy officers and more than 1,000 men.