The monarch’s investiture attire comprises the Supertunica, the Imperial Mantle, and the St Edward’s Crown (Picture: Getty/AP)

Britain’s new monarch will don heavy layers of golden robes for the exact moment he formally assumes the throne. 

During the ‘investiture’ – the portion of the coronation service in which the crowning takes place – King Charles III will wear a gold-sleeved coat known as the Supertunica, weighing around 2kg.

Over this, he will also wear a 3 to 4kg floor-length cloak called the Imperial Mantle, before the Archbishop of Canterbury places the 2.23kg St Edward’s Crown on the new monarch’s head.

The Supertunica, also known as the Close Pall of Cloth of Gold, is made of silk thread wrapped in thin pieces of gold.

It was created for Charles’ great-grandfather George V in 1911, and was also worn by his mother Queen Elizabeth II at her coronation in 1953.

The Imperial Mantle, or Robe Royal, meanwhile, is adorned with stylised arabesques and floral patterns, and was crafted for King George IV in 1921.

Charles will be assisted in assuming the heavy attire, which is usually kept at the Tower of London, by his son and heir to the throne, William, Prince of Wales. 

The gold-sleeved Supertunica weighs approximately 2kg (Picture: Victoria Jones-Pool/Getty Images)

The Imperial Mantle, which Charles will wear over the Supertunica for the investiture, weighs up to 4kg (Picture: Victoria Jones-Pool/Getty Images)

King Charles III will become the fifth British monarch of the House of Windsor when he formally assumes the throne on May 6 (Picture: Getty Images)

Caroline de Guitat, deputy surveyor of the King’s works of art for the Royal Collection Trust, said of the robes: ‘They have clearly incredible historic significance, but they’re also significant because of the sacred nature of their use during the investiture.

‘They are absolutely redolent of coronation ceremony.’

She added: ‘They have been worn at numerous coronations and His Majesty the King is following in this tradition of re-wearing these very ancient and historic garments.’

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By contrast, the Princess of Wales, Kate Middleton, will reportedly ditch the traditional royal look for the event on May 6, having decided to wear a flower garland rather than a tiara. 

Preparations are also being made for the first time the Crown Jewels will feature in public since Queen Elizabeth II’s coronation almost 60 years ago. 

In addition to these historic regalia, royal staff are polishing up ceremonial maces, the St Edward’s staff, and the 17th century Sword of State for use on the day.

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The monarch’s investiture attire comprises the Supertunica, the Imperial Mantle, and the St Edward’s Crown.