The Queen has become the first monarch for at least a century to attend Cabinet, taking 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.
Accompanied there by Mr Hague and the FCO's top civil servant Simon Fraser, she will meet staff and be presented with a memento of her overseas state visits.
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.
The Prime Minister's spokesman said: "The Queen will be visiting Downing Street and she will be presented with a gift from the Cabinet to mark her Diamond Jubilee year.
"She will then attend Cabinet as an observer. I think she will sit round the table and I think she will sit alongside the Prime Minister."
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 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".