The teenager joined his parents at St George's Chapel and it was the first time he has been seen publicly since being granted a new royal title.
Formerly Viscount Severn, James has taken over his father Prince Edward's former title, the Earl of Wessex.
Edward and Sophie became the Duke and Duchess of Edinburgh on the Prince's 59th birthday last month.
Buckingham Palace previously said that Edward would eventually one day succeed his father as the Duke of Edinburgh – but not until after the death of both Prince Philip and Queen Elizabeth II.
While the Earl of Wessex is a hereditary title, the Duke of Edinburgh is not. As such, at the time of his passing, Edward's role will not be passed on to his children. Instead, it will revert back to the Crown.
However, when that time comes, James is expected to then become the Earl of Wessex and Forfar – his father's Scottish title.
James was born on 17 December 2007 at Frimley Park Hospital in Surrey. He is above his older sister, Lady Louise Windsor, in the line of succession as The Succession to the Crown Act (2013) had not been created at the time of Lady Louise's birth in 2003.
The Act only applies to those born after 28 October 2011, including Princess Charlotte who retained her place in the line of succession after the birth of her younger brother, Prince Louis, in 2018.
Lady Louise, 19, was noticeably absent from the Easter Sunday celebrations because she is currently studying at the University of St Andrew's in Scotland.
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