Buckingham Palace has opted to refer to the new Prime Minister as "Andrew Burnham" in official records despite his well-known preference for "Andy."
During the Makerfield by-election that he used to unseat Sir Keir Starmer, Mr Burnham dropped both his party label and his surname from campaign posters, which simply read "Vote Andy."
Tuesday's Court Circular recorded: "His Majesty received in audience the Rt Hon Andrew Burnham MP and requested him to form a new Administration."
The decision is inconsistent with the precedent set by Tony Blair, whom the Palace referred to as "Tony" rather than "Anthony" throughout his premiership.
No explanation has been given for the different treatment.
The Court Circular serves as the definitive official log of all engagements carried out by working members of the Royal Family, making the naming choice a matter of permanent historical record.
It is not known whether Mr Burnham, who was raised in a suburb of Liverpool before growing up in Culcheth, Cheshire, will embrace being called "Andrew".
His team have been keen to emphasise just how relaxed his premiership will be, noting that he intends to don a jacket and tie only when appearing in the House of Commons.
The Cambridge graduate's default outfit is a black T-shirt and jeans.
Formal naming conventions have caused friction between the royals and Labour before.
On Tony and Cherie Blair's first stay at Balmoral with Queen Elizabeth, Princess Anne addressed Cherie as "Mrs Blair."
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Lancashire-born Cherie recalled the moment in her memoir, Speaking for Myself, stating: "At one point in that first year, Princess Anne came over and said something that included 'Mrs Blair'.
"'Oh. Please call me Cherie,' I said. 'I'd rather not,' she replied. 'It's not the way I've been brought up.'"
Mrs Blair wrote that her relationship with the Princess Royal "went rapidly downhill and never recovered."
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