The Queen has been given a rousing rendition of Happy Birthday by Royal Mail staff - and jokingly suggested that she may have added to their post bags.
On the eve of her 90th birthday, the Queen toured her local Windsor postal depot, which was renamed in her honour, accompanied by the Duke of Edinburgh.
And as she met staff at the newly named Queen Elizabeth delivery office, the monarch suggested to Royal Mail Group chief executive Moya Greene that cards and gifts from well-wishers may have added to the huge number of items they deliver every year.
Thursday's birthday is one of many milestones in her record-breaking reign. She is already Britain's longest reigning monarch but on April 21 she becomes the country's first nonagenarian sovereign.
There was a party atmosphere outside the depot, with long-serving staff from across the country invited along with their family and friends, and all waved Union flags.
Two choirs made up of Royal Mail staff from London and another group of singers from Bristol sang Happy Birthday to the Queen as she left in bright spring sunshine.
The Queen's visit on her last day as an 89-year-old saw her commemorate the anniversary of another British institution - the postal service.
Five hundred years ago, in 1516, Henry VIII knighted Brian Tuke, the first Master of the Posts - a move that was the catalyst for the creation of the Royal Mail we know today.
Jan Warbuton, 60, whose delivers post to staff living at Windsor Castle, was introduced to the Queen.
She said: "When Moya said we deliver over a billion parcels a year, the Queen said 'I've probably added to that this week'."
Ms Greene gave a speech before the Queen unveiled a plaque to mark her visit to the delivery office and paid tribute to the monarch.
She said: "Your Majesty, the leadership and commitment to public service you have shown throughout your lifetime continues to inspire us all.
"It is our privilege to carry your cypher on our iconic post boxes and your image on our postage stamps, each of which receives your personal approval before they are issued.
"Ma'am, we are enormously grateful to you and the Duke of Edinburgh for gracing us with your visit today on the eve of a significant milestone of your own.
"It is typical of you both to put service before self in recognition of this unique and special day in Royal Mail's history. I am delighted to announce that from today the Windsor delivery office will be known as the Queen Elizabeth delivery office.
"On behalf of all of Royal Mail's employees across the UK and here in Windsor, may I wish you a very happy birthday for tomorrow."
Ms Greene added to laughter from the gathered staff and senior managers: "I have it on good authority that your own postmen and women will be especially busy with tomorrow's mailbag."