For the past few years, the Queen has been the senior royal at the opening of the Field of Remembrance commemoration.
The annual event sees people placing memorials to those who have lost their lives while serving in the armed forces.
The Queen will also not attend a Buckingham Palace reception for Olympic and Paralympic athletes, which will be hosted King Charles.
But palace officials hope that she will be back for Remembrance events this weekend, which will include the commemorative service on Sunday morning at the Cenotaph in Whitehall.
The statement from the Palace says: “Her Majesty The Queen is currently unwell with a chest infection, for which her doctors have advised a short period of rest.
“With great regret, Her Majesty has therefore had to withdraw from her engagements for this week, but she very much hopes to be recovered in time to attend this weekend’s Remembrance events as normal.”
The Queen and King recently returned from a royal tour in Australia and Samoa, which included a summit of Commonwealth leaders.
Queen Camilla has also contributed to a documentary raising awareness about domestic violence, which will be broadcast next week.
King Charles received a cancer diagnosis in February. His treatment was paused during their overseas trip but was expected to begin again on his return.