For more than half a century, D-Day and the military invasion known as Operation Overlord have been synonymous with the Allied victory in the Second World War.

• Officers ordered D-Day and Overlord to be renamed Halcyon and Hornpipe.
• It was feared the original top secret codenames had been compromised.
• The new names have been discovered in never-before-seen documents.
• Son of army officer Sam Wheeler found them when clearing parents' home.
• Richard Wheeler then showed them to an expert on Antiques Roadshow.

But new never-before-seen documents have revealed the codenames for these campaigns were changed almost three weeks before the invasion of Nazi-occupied France.

The top secret documents - discovered among the possessions of a late Army officer involved in the planning of D-Day - were discovered when the officer's son presented his father's belongings to an expert on Antiques Roadshow.

The urgent memo was sent to senior officers on May 19, 1944, 18 days before the June 6 raid, stating: 'Overlord should be swapped for Hornpipe and D-Day should be referred to as Halcyon.'

The memo also said the codename 'Ripcord' should be used to signal a 24-delay to the operation.

The last-minute change came following concerns the original codenames had been compromised when many of them, including Overlord, appeared in crosswords in a national newspaper.

The Allied high command had referred to the the planned mass invasion of occupied France by the codename Overlord ever since it was conceived in 1943.

The remarkable development was discovered when Richard Wheeler was clearing out his late father Major Sam Wheeler's house and stumbled upon the documents.

Major Wheeler had helped plan the logistics for D-Day, which was masterminded by Field Marshal Bernard Montgomery, and had somehow ended up keeping the papers, stamped 'top secret'.

Mr Wheeler took them to show experts at the BBC's Antiques Roadshow at Durham Cathedral - and they were left stunned when he explained what they were.

Mr Wheeler, 67, from Durham, said: 'My father joined the army at the outbreak of war and worked his way up through the ranks, eventually joining the 21st Army Group in 1944.

'They were based at the army's headquarters in London and was responsible for the planning of the D-Day invasions.

'My father was involved in the logistics of the invasion and so would have been close to these messages being passed back and forward... (see more at: http://www.dailymail.co.uk/)