Senior royal officer Lord Chamberlain, the Queen's private secretary Sir Christopher Geidt will address aides at 10pm, the Daily Mail reports.
There is intense speculation as to what the meeting is about and a huge media presence has already assembled at the palace.
But rumours of a death or announcement of an abdication were tempered by a source who spoke to Reuters saying: "There's no cause for alarm."
Buckingham Palace did not comment when contacted by HuffPost. It is believed anything newsworthy stemming from the all-staff meeting will be announced later.
Prince Philip was at Lord's cricket ground yesterday and appeared in good health.
Meanwhile the Queen hosted Theresa May at Buckingham Palace.
And according to one Canadian journalist the Palace has confirmed both the Queen and Prince Philip are "alive and well".
HuffPost UK's Head of Video, Dawn Kelly, reported from Buckingham Palace: "There's a quiet sense of anticipation - more cameras are setting up.
"No one seems to know anything but news presenters are poised in front of their cameras, ready to go live.
"There are seven satellite trucks and many confused tourists. The gardeners seem bemused by the whole thing."
Royal staff members from as far afield as Balmoral in Scotland are reported to be attending the meeting.
One told the Daily Mail: "Everyone is on tenterhooks.
"Although meetings involving the entire royal household are occasionally called, the way this has been done at the eleventh hour is highly unusual and suggests that there is something major to be disseminated."
Buckingham Palace had no comment on the Mail report but the source said such all-staff meetings did occur occasionally.
ITV's Chris Ship said in a Facebook post: "...there is no imminent announcement of a death - as some have claimed - and nor is the Queen about the say she is abdicating.
"Beyond that, I can't say much more."
The Queen turned 91 last month while her husband, Prince Philip, will celebrate his 96th birthday in June.
Both cancelled a number of engagements over the Christmas period due to "heavy colds".
Reports of "no cause for alarm" didn't stop many people worrying anyway.