King Charles personally intervened to ask Prince Harry to wear his uniform after he overturned the ban.
He will wear it on Saturday in a special tribute to his late grandmother the Queen.
The Duke of Sussex was given special permission to put on his military uniform for a lying-in-state vigil staged by Her Majesty’s grandchildren at Westminster Hall.
Harry and his brother Prince William, will be among eight grandchildren who will stand in silence on Saturday evening for the 15 minute vigil alongside Her Majesty’s coffin.
The King spoke directly and personally to both of his sons to convey his wish that they should both wear their military uniforms during Saturday’s vigil.
The Mirror exclusively revealed on Thursday that it would be the first time Prince Harry would be granted permission to wear his Blues and Royals uniform once again, despite no longer being a working member of the Royal Family.
To spare his embarrassment, and that of Prince Andrew, at the funeral of his late grandfather Prince Philip, the Queen decided that all the royal men would wear morning suits instead of their military uniforms.
But the King in this instance personally declared that his sons should don their outfits as “it was the Queen’s long held wish” that William and Harry would be dressed in their military uniforms.
A spokesman for Prince Harry has insisted he did not make a plea to his father to wear the uniform, it is understood.
A royal source said: “It is of no surprise that the King intervened in this way.
“He has the utmost respect for both his sons and their dedication to their military careers.
“The Duke of Sussex will wear his uniform as a mark of respect for his beloved grandmother but also the Queen and country he served for a decade.
“All the family believe this is the just and right decision, of course.”
It will be the first time Prince Harry would be allowed wear his Blues and Royals uniform once again, despite no longer being a working member of the royal family.
But the King insisted his sons should wear their outfits as “it was the Queen’s long held wish” they should be dressed in their military uniforms.
Prince Harry insisted he did not make a plea to his father to wear the uniform, it is understood.
A spokesperson for the duke said: “Prince Harry will wear a morning suit throughout events honouring his grandmother.
“His decade of military service is not determined by the uniform he wears and we respectfully ask that focus remain on the life and legacy of Her Majesty Queen Elizabeth II.
Harry, who saw action on the front line during two tours of duty in Afghanistan, has been denied the chance to wear his military uniform, as he publicly mourns, because he is no longer a working royal.
Despite being a former Army officer, he has been in civilian dress for official events, including walking behind his grandmother's coffin on Wednesday when it was carried to Westminster Hall for lying in state.
An exception had been made for the Duke of York, who is no longer a working royal but will wear his military uniform as a "special mark of respect" for the Queen when he stands guard around her coffin during a vigil with his siblings on Friday evening.
During Saturday's vigil, the Prince of Wales will stand at the head and the Duke of Sussex at the foot. The other grandchildren will be in morning coat and dark formal dress with decorations.
William will be flanked by his cousins Zara Tindall and Peter Philips, while Harry will be with Princesses Beatrice and Eugenie, with Lady Louise and Viscount Severn at the centre of the coffin.
The source said the grandchildren, at the King's invitation, were very keen to pay their respects - just as their parents are doing on Friday evening.
