Queen Elizabeth II is poised to become Britain's longest-reigning monarch. On 9 September she will beat the record held by her great-great-grandmother, Queen Victoria, who reigned for 63 years and 216 days.

As the eldest child of the Duke and Duchess of York, Elizabeth Alexandra Mary would have been expected to play merely a supporting role in the monarchy. At the time of her birth on 21 April 1926, her father was only second in line to the throne. His brother, the Prince of Wales, was heir apparent to George V. Elizabeth's life changed abruptly on 11 December 1936, when her uncle, now King Edward VIII, announced he was renouncing the throne to marry twice-divorced American socialite Wallis Simpson. The crown passed to his brother, who became George VI.

In February 1952, Princess Elizabeth and her husband Prince Philip were on a tour to Kenya when news of King George's death reached the royal couple. The princess returned to London as Queen Elizabeth II. Her coronation, at the age of 27, took place in Westminster Abbey on 2 June 1953. The ceremony was broadcast by television and radio throughout the world.

Queen Elizabeth's youth and gender caused some comment at the time, but turned out to be an asset in a country keen to make a fresh start after the war and years of austerity which followed. Social divisions and respect for the old order began to wane but the Queen was seen as a symbol of unity.

In July 1969, Prince Charles was invested as Prince of Wales at a ceremony in the 700-year-old Caernarfon Castle. During the ceremony the Prince, following ancient tradition, pledged his loyalty to the Queen before being vested with the emblems of office.

In 1981, 32-year-old Prince Charles married 20-year-old Princess Diana in a ceremony watched by an estimated worldwide television audience of 700 million people. Princess Diana gave birth to the couple's first son, William, in June 1982. Their second son, Harry, was born in 1984. In March 1986 Prince Andrew, the Queen's second son, announced his engagement to childhood playmate Sarah Ferguson, who brought with her what was initially seen as a breath of fresh air into a royal family hidebound by tradition.

The following decade saw the royal family under the public spotlight as never before. The Queen referred to 1992 as an "annus horribilis", or horrible year. Princess Anne's 18-year marriage ended in divorce amid speculation about her relationship with her future second husband Royal Navy Commander Timothy Laurence; Prince Andrew and Sarah Ferguson announced their formal separation after six years of marriage; the Queen's favourite royal home, Windsor Castle, was badly damaged by a fire; the perilous state of Prince Charles' and Princess Diana's marriage was revealed in the biography: "Diana, Her True Story", and in December they announced their separation.

The years following the break-up of her marriage saw Diana's popularity soar and thousands mourned her passing after she was killed in a car crash in Paris on 31 August 1997. The Queen came under unprecedented pressure to recognise the nation's grief and pay public tribute to the "people's princess".

During her reign, the Queen has broken new ground for the British monarchy during many of her overseas visits. She was the first British monarch to visit Japan. She met Chinese leader Deng Xiaoping and became the first British monarch to walk on the Great Wall of China. In October 1994 the Queen arrived in Moscow for a ground-breaking visit aimed at healing nearly five decades of Anglo-Russian Cold War enmity.

On New Year's Eve 1999, the Queen took part in London's New Millennium celebrations by joining hands with Prime Minister Tony Blair and singing Auld Lang Syne as thousands of revellers watched a spectacular fireworks display.

Another milestone was marked in August 2000 when the Queen joined her mother and her sister on the balcony at Buckingham Palace to cheers from the crowds gathered to celebrate the Queen Mother's 100th birthday.

The celebrations were soon to be followed by a period of loss and mourning for the Queen. Princess Margaret died in February 2002 after suffering a number of strokes and two months later the Queen Mother died, at the age of 101.

Less than a month after the Queen Mother's funeral the Queen began a gruelling Golden Jubilee tour marking her 50 years on the throne which culminated in four days of nationwide celebrations in June 2002.

After a 35-year affair Prince Charles finally married the love of his life, Camilla Parker-Bowles, in April 2005. As titular head of the Church of England the Queen declined to attend the civil ceremony in Windsor Guildhall but together with Prince Philip and the rest of the royal family attended the service of blessing in St George's Chapel, Windsor.

She has shown little sign of slowing down in her ninth decade. She welcomed newly-elected US President Barack Obama and First Lady Michelle Obama to Buckingham Palace. During her reign the Queen has met all 12 serving US presidents except Lyndon B Johnson.

The wedding of the Queen's grandson Prince William to Kate Middleton took place on 29 April 2011 in Westminster Abbey. The ceremony was watched by an estimated two billion people around the world.

In May 2011 the Queen made a State visit to the Republic of Ireland, the first by a British monarch since Ireland won independence in 1921.

On the 60th anniversary of the Queen's accession to the throne and the beginning of her Diamond Jubilee celebrations. The Queen and Prince Philip spent six months taking part in Jubilee events across the United Kingdom.

The celebrations included a spectacular flotilla of 1,000 vessels taking part in the largest display of pageantry ever seen on London's River Thames for 350 years. Typical British weather failed to dampen the enthusiasm of the thousands of well-wishers taking part or watching from the river embankments.

The Queen had special cause for celebrations in June 2013 when her horse, Estimate, won the Royal Ascot Gold Cup. It was the first time a reigning monarch's horse had claimed victory in the race's 207-year history. The Queen had been expected to present the winner's trophy, but instead received the cup from Duke of York, Prince Andrew.

With the birth of the Duke and Duchess of Cambridge's son in July 2013, for the first time in over a hundred years there were three generations of direct living heirs to the throne. Prince George, named after the Queen's father, became third in line to the throne, after his father and grandfather.

The Queen's first prime minister, Winston Churchill, served in Queen Victoria's army and was born in 1874. Current prime minister, David Cameron, was not born until 1966 and the Queen first saw him as a small boy dressed as a rabbit in a school play her son Edward was appearing in.

Events marking the centenary of the outbreak of World War One were held worldwide in 2014, and the sea of red ceramic poppies installation 'planted' in the dry moat of the Tower of London attracted an estimated five million visitors. The Queen and Prince Philip visited the site in October. Each of the 888,246 poppies commemorates a British or Colonial soldier who died in the four-year conflict.

Princess Charlotte, the Duke and Duchess of Cambridge's second child and the Queen's fifth great-grandchild, was christened at a small, family service at the church on the Sandringham estate.

During her reign the Queen's personal approval ratings has remained high in spite of the occasionally strong criticism attracted by other members of the royal family. The 89-year-old monarch is seen as a dignified figure, much loved by royalists and respected even by republicans.

Elizabeth once said: "it's a job for life", and unlike some recent European monarchs – and even a pope – she is not expected to abdicate. That means Prince Charles, 66, already a record-holder himself as Britain's longest-serving heir apparent, will have to wait longer still before he becomes king.

Aides say the Queen wants little fuss over this milestone. She will be in Scotland, holidaying at her Balmoral Estate and will undertake an official engagement on the day itself with First Minister Nicola Sturgeon. Buckingham Palace said the Queen would rather save the public celebrations for next April, when she turns 90.