The Queen's attendance at Cabinet on Tuesday is "daft" and "inappropriate", according to a leading academic.
Rodney Barker, emeritus professor of government at the London School of Economics and Political Science, suggested the move would blur the boundaries between government and the monarchy.
The Queen will become the first monarch to attend the meeting since Queen Victoria and will sit between Prime Minister David Cameron and Foreign Secretary William Hague.
She will be presented with a gift to mark her Diamond Jubilee, paid for through contributions by each Secretary of State, before observing the weekly briefing.
Although the Queen performs some ceremonial and formal duties relating to government, including the state opening of Parliament, the head of the state must remain strictly neutral on political matters.
Prof Barker said he believed the move was "inappropriate", adding: "I think it is daft, it muddies the waters.
"It will mean potentially the Queen will know things she is not supposed to know and hear things she is not supposed to hear."
He said: "Cabinet meetings, on the whole, are to confirm what has already been agreed but there is some sort of discussion. Presumably tomorrow they are all going to sit there agreeing and nodding their heads."
Prof Barker said the Queen would not be able to make any contribution because the role of head of state was "totally apolitical".
He said it was a surprising decision by Buckingham Palace but added that "they must have their reasons".