The Queen surpassed her great-great-grandmother Queen Victoria’s long reign in 2015.
When a member of the Royal Family dies, there is a formal process to be followed – here’s what will happen next.
Though of course the Queen will be grieving for her loss in her own way, the royal will now enter an official mourning period of eight days.
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As soon as the death was confirmed, members of the Royal Family and representatives of the Royal Family would have been expected to wear black or dark colours and mourning bands, according to tradition.
Once the funeral has passed, the Queen will continue to mourn in private, but she will resume her royal duties from behind closed doors.
However, the full period of mourning for the Royal Family will continue for a further 30 days.
It is only after this time that the Queen will make a full return to public life and duties.
Flags will be lowered to half-mast to mark the passing of the Duke of Edinburgh, with the exception of the Royal Standard flag above Buckingham Palace.
The flag represents the monarchy and is never lowered even in the event of a royal death.
Government ministers will also show their respects for the late Prince Philip.
Despite the sombre time, social engagements will be cancelled, but official engagements may still go ahead for some royals.
However, while this is the official protocol, there’s no guarantees that the Queen will follow this exact process.
Previous monarchs have mourned in their own ways, not necessarily following the traditional time period to grieve for their spouse.
However, it’s thought that Queen Elizabeth will likely stick to the schedule and resume her duties as planned, though it will understandably be difficult after her long and happy marriage.
There may also be a nationwide two minute silence to mark the death of the Duke.
The deaths of both George VI and Queen Elizabeth The Queen Mother were marked by a national two minute silence on the days of their funerals.