Bonhams will mark the 70th anniversary of D-Day by selling the secret plans for Mulberry harbour - a temporary port that proved vital to the Allied landings.

The Mulberry harbours made the Allied D-Day landings possible, destroying two years of defence work by the Nazis

Valued at £40,000-60,000 ($67,807-101,210), the pencil sketches were created by engineer Hugh Iorys Hughes, who proposed the idea of a floating harbour to remedy the need for a port big enough to support the European attack.

Hughes first pitched the idea to the War Office in 1941 and built a prototype in Conwy, Wales to convince the authorities. It was his brother, a well connected officer in the Royal Navy, that first alerted senior staff to the ingenious idea.

According to Bonhams, Winston Churchill instantly warmed to the scheme as he had proposed a similar plan during the first world war.

"Piers for use on beaches. They must float up and down with the tide. The anchor problem must be mastered. Let me have the best solution worked out. Don't argue the matter. The difficulties will argue for themselves," he wrote to Admiral Mountbatten.

The remains of the Mulberry harbour, which can still be seen at Arromanches, Normandy today

Such was the success of the Mulberry harbour that Albert Speer, the Nazi architect behind some of the second world war's most effective defences, told the Nuremberg Trials:

"To construct our defences we had in two years used some 13 million cubic metres of concrete and 1½ million tons of steel. A fortnight after the Normandy Landings, this costly effort was brought to nothing because of an idea of simple genius.

"As we now know, the invasion force brought their own harbours, and built, at Arromanches and Omaha, on unprotected coast, the necessary landing ramps."

See our superb selection of first and second world war memorabilia for sale, including signed items from Winston Churchill.