Wrecks and ruins that are among the last vestiges of the great engineering achievements for D-day have been given heritage protection before the 75th anniversary of the Normandy invasion on 6 June.

They include six concrete structures built as replica landing craft for troop training, and sunken tanks lost along with crew during assault rehearsals that went badly wrong off the south coast of England. Portions of the famed Mulberry floating harbours are also to be protected.

During the final preparations for D-day, General Dwight D Eisenhower, the supreme Allied commander, described the southern portion of England as “one vast camp, dump and airfield”.

Six weeks before 6 June 1944, troops rehearsed an assault codenamed Exercise Smash at Studland beach, Dorset, watched by Eisenhower, Winston Churchill and King George VI. Seven army tanks known as Duplex Drive (DD) Valentine tanks, modified to be amphibious tanks so they could leave their landing craft further from shore, were lost because of bad sea conditions. Six crew members died.

The wreck of a Valentine tank off Poole. Photograph: Paul Pettitt

A valuable lesson was learned, and on D-day the tanks were released only in shallow water. Today, the Valentine tanks in Poole Bay, which have been awarded scheduled monument status, represent the largest surviving group of their type anywhere in the world.

Also given monument status – reserved for nationally important sites – are two Centaur cruiser tanks, designed to fire at concrete targets such as pillboxes, and two armoured bulldozers, designed to clear the invasion beaches of obstacles, which were lost when a 286-ton landing craft capsized off Selsey Bill, West Sussex, en route to Normandy on D-day.

Quick Guide What happened on D-day? Show What was D-day? D-day was an invasion of France by allied forces. It was codenamed Operation Neptune, and it aimed to push Nazi Germany out of occupied France. Five beaches in Normandy, codenamed Omaha, Utah, Juno, Sword and Gold, were the main targets for landing a large number of troops by sea. At 10pm on 5 June 1944, troops began departing from British shores to head across the Channel. Five assault groups set sail under darkness in an armada of about 7,000 vessels. Just after midnight on 6 June, aerial bombardment of enemy positions on the Normandy coast began. Special operations troops were also parachuted into France. US troops landed on Omaha and Utah beaches at about 6:30am. About an hour later Canadian forces landed at Juno, and British troops landed at Gold and Sword. Soldiers had to get off their boats, wade through the water, and seize control of the beach, all the while under heavy and sustained fire from German defensive positions. How was the plan kept secret? Despite involving a large number of troops, keeping D-day secret was vital to the success of the operation. A disinformation campaign had led the Germans to believe that Operation Fortitude was the main plan for the allies to invade the continent, via a two-pronged attack involving Norway and Calais. Even once the D-day landings had begun, German commanders were convinced they were just a diversionary tactic before the real invasion. Why is it called D-day? The D in D-day actually has no particular significance to Operation Neptune. It was common practice in the military to make plans that used the term, where the D stands for the day when operations commenced. Military planners also set H-hour, the time at which a plan was to begin. What happened next? By the end of the day, the allies had disembarked more than 135,000 men and 10,000 vehicles on to the beaches, and established bridgeheads of varying depths along the Normandy coastline. This came at the cost of 4,400 allied troops being killed, with thousands more injured or missing. There were also heavy casualties among German troops and French civilians. By 19 August, the allied forces had pushed down far enough to begin the battle to liberate Paris. German troops surrendered the French capital on 25 August 1944, two and a half months after D-day. Martin Belam

Components of the ingenious artificial Mulberry harbours, made in huge sections, towed across the Channel, and assembled off the beaches so the allies could rapidly land supplies and equipment, are also being saved off Pagham Harbour, West Sussex, after being damaged during stormy weather before being towed to France.

Replica landing craft in Braunton Burrrows, Devon. Photograph: James O Davies/Historic England Archive, James O. Davies

Troops had to rehearse how to get on and off landing craft at speed, and for this purpose special replica landing craft installations were made from concrete, designed to represent the top deck of the craft with its front ramp down. Six of these structures, used for training American troops at Braunton Burrows in Devon, have now been given Grade II listing.

The culture secretary, Jeremy Wright, said: “By listing the landing crafts, tanks, bulldozers and floating harbours we can ensure that future generations can learn about this important moment in our history.”

Duncan Wilson, the chief executive of Historic England, said: “Evidence of D-day planning, rehearsal and the actual operations is all around us, on our coastline and in our waters, helping to tell the D-day story.”

Historic England and Wessex Archeology have been carrying out underwater surveys of two large American vessels, known as Landing Ship, Tanks (LSTs), that were sunk as part of the ill-fated Exercise Tiger D-day practice landings at Slapton Sands in Devon.

The ships were carrying hundreds of American servicemen as well as tanks, vehicles and trucks, and were torpedoed as a result of nearby German E-boats intercepting their communications, resulting in the deaths of more than 600 men. It is hoped the ships can be granted protection later this year.