The Queen is to become the first monarch since Queen Victoria to attend a Cabinet meeting in Downing Street.
Number 10 has revealed that Queen Elizabeth II will observe the ministers' meeting on Tuesday morning.
It is expected that she will sit next to Prime Minister David Cameron for the summit, which is the last before the Christmas break.
She is not due to stay for the whole session - likely to last around 90 minutes - and will instead watch for around half an hour.
It is also understood that Prince Philip will not accompany her.
Mr Cameron announced the unusual move on Twitter, writing: "The Queen will visit Downing Street tomorrow.
"She will be presented with a gift to mark her Diamond Jubilee and attend Cabinet as an observer."
Downing Street refused to give any details about the gift the Queen will receive during what it described as a "long-planned" visit.
But a spokesman said: "I think she will sit round the table and I think she will sit alongside the Prime Minister."
Energy Secretary Ed Davey told Sky News: "It is a great honour that she is attending Cabinet in her Diamond Jubilee year.
"It has been a very exciting year. Ministers will be excited but minding their Ps and Qs even more than normal."