LONDON — Queen Elizabeth II, already Britain’s longest-serving monarch, passed a new milestone on Monday, together with Prince Philip, as they became the longest-married couple of the royal family.

The family released a series of new portraits taken earlier this month of Elizabeth, 91, and Philip, 96. The two appeared against a platinum-textured backdrop, a hint at the 70th anniversary, in the White Drawing Room at Windsor Castle. One of the images showed them alongside portraits of another long-lasting couple, King George III and Queen Charlotte, who were married for 57 years.