'Chastity' was an Allied unrealised plan to place an artificial harbour of the 'Mulberry' type in Quiberon Bay on the north-western tip of France to support land operations to clear the Germans forces our of Brittany (summer 1944).

The plan was not implemented as, by the time the surrounding territory had been taken, the front had moved several hundreds of miles to the east from the Normandy lodgement secured in 'Overlord' and the great port city of Antwerp had been taken with its facilities intact.

A vital factor in the 'Overlord' concept had been the rapid capture of deep-water ports, for these were seen as essential to the handling of the large numbers of reinforcements and vast tonnages of equipment and supplies needed for the furtherance of the campaign. 'Chastity' was devised in April 1944 to satisfy this requirement through the rapid construction of a complete new port in Quiberon Bay, and was the final major revision to the invasion plan.

Quiberon Bay is a large anchorage, between Lorient and St Nazaire on the south-western coast of Brittany, sheltered by the Quiberon peninsula and a line of small islands and containing four small ports. The Auray river drains into the bay near one of the ports, Locmariaquer, and had scoured a pool some 3,000 yards (2745 m) long and between 30 and 300 yards (27.5 and 275 m) wide, with nearly vertical sides and a depth of 80 ft (24.5 m), just offshore.

The implementation of 'Chastity' would have seen the construction of floating piers in the pool, allowing large ships to tie up alongside, with bridges to carry the cargo and troops to the shore. The plan was berths for the simultaneous offloading of five ships, providing a capability of 2,500 tons of supplies per day directly onto vehicles. A further 7,500 tons per could be offloaded using lighters carrying supplies directly to the shore from 30 more ships moored in the pool.

This was seen as a very efficient scheme, since the two 'Mulberry' prefabricated ports constructed on the Normandy beaches provided 6,000 tons of supplies a day at a construction cost of 120,000 man months, while the 'Chastity' facility would have provided 10,000 tons per day by comparison with 26,000 tons per day through Normandy and Cherbourg) but would only need 4,000 man months for the construction of the prefabricated facilities. A further advantage was the ready access to local rail facilities and the relatively undamaged rail network away from the Normandy region.

'Chastity' was posited on the early capture of Brest and Lorient, both of these being designated as fortresses by Adolf Hitler, since shipping would be liable to attack as it passed these German-held ports on the way around the peninsula from the UK. The plan was approved on 22 April 1944 and the capture of Quiberon Bay was thereupon accorded a high priority. Cherbourg was the first major objective of the invasion, its capture originally scheduled for 14 June (D+8) but moved back to 21 June just before the invasion. However, it was captured only on 27 June. The capture of Breton ports such as St Malo was further delayed, and the assault on Brest did not start until 25 August. By the time that Quiberon Bay was on the point of being captured, the Allies had taken Antwerp (on 4 September) with its port facilities intact.

On 9 September, General Dwight D. Eisenhower, commanding all the Allied forces in North-West Europe, abandoned all plans for further exploitation of Brittany ports, including 'Chastity', which were now seen as irrelevant as they were now hundreds of miles from the front of the Allied advance.