By Dan Bilefsky

New York Times

LONDON >> She has been served by 12 prime ministers, starting with Churchill; navigated the decline of the British Empire; braved the tragedies of her family and the nation; and, on Sept. 9, edged out Queen Victoria as the longest-reigning monarch in British history: 64 years now. She is lauded for having the stiffest upper lip in the realm.

On Thursday, Queen Elizabeth II will celebrate her 90th birthday, and a grateful nation will honor a woman her biographer Douglas Hurd, a former foreign minister, has called “The Steadfast.”

Indeed, through seven decades, she has remained gloriously and relentlessly enigmatic in one of her signature pastel outfits and colorful hats, chosen, royal experts say, so onlookers can spot her in a crowd.

This being Britain, the occasion will be celebrated with pageantry; warm beer; longer pub hours; equestrian displays; and an appearance by actress Helen Mirren, who won an Oscar for portraying the queen.

But, befitting a workhorse who carried out 341 engagements last year, Elizabeth kicked off birthday celebrations on Wednesday in a dutiful display. At a Royal Mail delivery center, where she and her husband, the Duke of Edinburgh, watched workers sort mail and were serenaded by a choir of postal employees. On Thursday, she is to light the first of more than 900 celebratory beacons. On Friday, President Barack Obama will offer the queen birthday wishes at Windsor Castle.

And the frenzy won’t end this week.

Pub opening times in England and Wales will be extended for two hours, until 1 a.m. on June 10 and 11. (Her birthday is officially celebrated in June for ceremonial purposes.) A British artist has also paid tribute to her service as an Army mechanic during World War II by using 800 car parts to create a giant sculpture of the queen’s head, including a crown made with spark plugs. There is a new set of postage stamps to honor the birthday, featuring four generation of royals, including little Prince George. They will be first class, of course.

Retailers and canny subjects are cashing in. Those who cannot afford the bespoke 22-karat gold “God Save the Queen” music box (retailing for $1,425) can buy a souvenir waistcoat emblazoned with the queen’s image ($50, on eBay).

In a nod to Britain’s increasing multiculturalism, Nadiya Hussain, a headscarf-wearing Muslim baker who won the BBC’s wildly popular “Great British Bake Off,” will prepare an orange drizzle cake with orange curd and orange buttercream. (A 68-year-old slice of the queen’s wedding cake fetched $719 at a September auction.)

Vernon Bogdanor, an eminent constitutional scholar at King’s College, London, commended the queen for keeping her views to herself, arguing that the enigma of her persona has been essential to her success at symbolizing and unifying the nation.

“She grew up during the war when people had to show emotional restraint,” he said. “Not like today when you have to let everything out.”

Bogdanor said he could recall only two gaffes by the queen: The first came on a trip to St. Petersburg, Russia, in 1994, when she evidently told a young Russian biologist, who had studied in Manchester, that the city was “not such a nice place.” (Buckingham Palace denied it at the time.) More memorably, in 1997, the queen misread the public mood after Princess Diana died in a car crash in Paris, and initially remained in Balmoral, Scotland, with her grandsons, rather than returning to London to emote before a grieving nation.

Retaining the sparkle of a creaking institution is not easy. Britons also recently got a rare glimpse inside the royal household in an ITV documentary “Our Queen at 90,” in which her subjects learned that her 2-year-old great-grandson, Prince George, calls her “Gan-Gan.” His father, Prince William, second in line to the throne, noted that some people are so overwhelmed when they meet his grandmother that they faint.

Born on April 21, 1926, Elizabeth was initially third in line to the throne. In 1947, she defied her parents to marry Prince Philip, a part Danish, part Greek, prince she was smitten with when she saw him at age 13. She acceded to the throne in 1952 after the death of her father, King George VI; his older brother, King Edward VIII, abdicated in 1936 to marry an American divorcée, Wallis Simpson.

Though the monarchy is associated with the class system, wealth and privilege, Peter York, a leading cultural commentator, argued that the queen’s unbridled blandness was a form of “human bondage.” Britons, he said, relished reports of her legendary thriftiness, including stories, apocryphal or not, that she roams Buckingham Palace turning off lights, enjoys soap operas and eats “non-banquet dinners out of Tupperware containers.” In private, she is said to have a keen sense of humor.

York added: “She should be congratulated for living this long. The certainty of her life reminds Britons of the continuity of their own.”

York said the queen could also display barely decipherable piques of annoyance, as when she asked in 1998, during a visit to the London School of Economics, why no one had foreseen the financial crisis, which had dented her wealth.

“Every so often you see a little lip-tightening that suggests, ‘What fresh hell is this?’ when she is forced to endure something hideous or idiotic,” York said.

The queen’s mother lived to 101, and Elizabeth shows little sign of slowing down, but Stephen Bates, the author of “Royalty Inc.: Britain’s Best-Known Brand,” said that in Buckingham Palace, uttering the word “abdication” was akin to swearing in a church.

He also expressed concerns that the queen’s line of presumed successors — Charles, Prince of Wales; Prince William, Duke of Cambridge; and Prince George — may lack her luster. “William has become just another middle-aged bald bloke in a suit,” he said.

Whatever the challenges, the queen, for her part, appears to have kept the promise she made on her 21st birthday: “I declare before you all, that my whole life, whether it be long or short, shall be devoted to your service.”