In contrast to the exuberant celebrations on her actual birthday on Friday, the Queen sat solemnly for the thanksgiving service at St George's Chapel in Windsor, the burial place of 10 monarchs, including her father George VI and Henry VIII.
The sermon in the grounds of Windsor Castle, a centuries-old royal retreat west of London, echoed messages uttered last week by British politicians and by Prince Charles, her eldest son and heir to the throne.
The Queen sat in the front row wearing a duck egg blue silk outfit with a matching hat. The Dean of Windsor David Conner told the congregation she had given "heart" to Britons through her selfless and steadfast devotion to duty.
Head of the Church
Bishop David added that the Queen's Christian faith was the "very bedrock" of her life and 54-year reign. She is head of the Church of England as well as head of state, the armed forces and the Commonwealth.
The Queen remained modest despite the limelight, he told the 700-strong congregation, including Prince Charles and Camilla, who received a blessing in the same chapel when they were married in Windsor just over a year ago.
About 150 members of the public gathered in the rain outside, far fewer than the 20,000 flagwaving and cheering wellwishers who flocked to her during her 45-minute walkabout in Windsor on Friday.
By highlighting her strengths and the role of the monarchy, Bishop David made a strong statement in support of an institution that has survived calls to abolish it, largely due to her own enduring popularity.
"And, in a world in which we often feel confused and buffeted by so many changes," her throne provided a sense of continuity while she personally conveyed "a sense of calmness, serenity and stillness," Bishop David said.
The Queen's attributes "help us to understand that real human beings need to be deeply rooted if they are to flourish", he said.
Britain's politicians, led by Prime Minister Tony Blair, last week paid homage to the Queen for providing a sense of continuity and stability to Britain and the 53-nation Commonwealth.
On her birthday on Friday, she received a glowing tribute from Prince Charles, who called her his "darling Mama" when he praised her steadfastness in her devotion to duty.
The solemn Sunday service was also in contrast to the melodies of Elvis and Bjork which blasted around Kew Palace for the dinner party and fireworks display in London on Friday.
Scandal
Queen Elizabeth also enjoys an official birthday, this year on June 17, when the fickle English weather is deemed more favourable for outdoor festivities.
Born on April 21, 1926, Elizabeth Alexandra Mary has grown to become one of the best-loved members of the House of Windsor.
She was proclaimed queen in February 1952, aged just 25, after the death of her father, King George VI, and was crowned in June
1953.
Meanwhile a whiff of scandal has returned to the monarchy with claims that the 61-year-old wife of a cousin of the Queen is having a fling with a 40-year-old Russian furniture tycoon.
In a front-page splash, The News of the World newspaper said Princess Michael, who has been married to Prince Michael of Kent since 1978, had a romantic break with long-haired Mikhail Kravchenko in Venice, Italy.
Simon Astaire, spokesman for the princess, insisted the pair were just friends and had met in Venice to discuss plans for new textile designs.