The Queen will become the first monarch for at least a century to attend Cabinet when she takes a seat beside Prime Minister David Cameron at the famous Number 10 table.
Arranged to mark her Diamond Jubilee, she will be presented with a gift paid for through contributions by each Secretary of State.
But although she is expected only to observe the discussions and not contribute to the debate, the move has been criticised by a constitutional expert. Rodney Barker, emeritus professor of government at the London School of Economics and Political Science, said it was "daft" and would blur the boundaries between government and monarchy.
At what is believed to be the first such visit since the reign of Queen Victoria, she will sit between Mr Cameron and Foreign Secretary William Hague. Officials said the Queen would leave before the end of the meeting to fulfil other diary commitments - including a visit to the Foreign and Commonwealth Office.
Downing Street refused to disclose any details about the gift she will be presented with but said all members of the Cabinet had contributed to it equally.
Although the head of state performs some ceremonial and formal duties relating to government, including the state opening of Parliament, they must remain strictly neutral on political matters.
Prof Barker said he believed the move was "inappropriate". "I think it is daft, it muddies the waters," he added. "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 ... 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".
Lord O'Donnell, former Cabinet secretary, told the BBC Radio 4 Today programme: "I'm sure Cabinet want to do this because they want to say thank you. I've always viewed the Queen as the ultimate public servant."
"The Queen, she did come and visit the Cabinet Office and sat in on a meeting of the perm secs (permanent secretaries) so we got there first," he added. "This is right for her to be doing this. I think it is exactly the right thing and it shows she would have done this whichever party had been in power."