On the lawns of the palace, thousands of people stood silently in tribute to the 22 people, including children, killed in the blast and the 59 injured.
The Queen is due to be joined at the garden party by the Duke of Edinburgh, Prince of Wales, Duchess of Cornwall, Duke of York and Princess Eugenie.
In a message of support released earlier, the Queen said:
'The whole nation has been shocked by the death and injury in Manchester last night of so many people, adults and children, who had just been enjoying a concert.
I know I speak for everyone in expressing my deepest sympathy to all who have been affected by this dreadful event and especially to the families and friends of those who have died or were injured.
– QUEEN'S MESSAGE OF SUPPORT