The tradition goes back hundreds of years as a sign of mourning and respect.
The Government’s Digital, Culture, Media & Sport and Cabinet Office issued official guidance on flag protocol shortly after the death of the Queen.
It offers advice to organisations wanting to pay their respects, saying flags should remain at half mast until after the Queen’s funeral.
A statement posted on the department’s official website states: “All official flags, including the Union Flag, should be half-masted until 8am on the day following The Queen’s State Funeral.
“Flags may be flown overnight during this period but should remain at half-mast.
“Official flags in this instance are defined as national flags of the home nations, Crown Dependencies and Overseas Territories, Ensigns and Ships’ colours.
“Any non-official flags flying or due to be flown, such as the Rainbow Flag or the Armed Forces Day Flag, should be taken down and replaced with a Union Flag at half-mast. Other official flags scheduled to be flown can be flown as normal, but at half-mast.
“Half-mast means the flag is flown a third of the way down the flagpole from the top, with at least the height of the flag between the top of the flag and the top of the flagpole.
“On poles that are more than 45 degrees from the vertical, flags cannot be flown at half-mast and should not be flown at all. The Union Flag must be flown the correct way up – in the half of the flag nearest the flagpole, the wider diagonal white stripe must be above the red diagonal stripe.
“The Royal Standard is never flown at half-mast even after the death of a monarch, as there is always a Sovereign on the throne and it would therefore be inappropriate for it to fly at half-mast.
“The Union Flag will be flown at half mast on all Royal Residences.”