Britain's politically neutral head of state has attended the normally politics-riven Cabinet meeting which guides government thinking for the first time in her reign.
The monarch, who has been shut out of political decision-making for centuries, came under fire from some experts for a move which some claimed "muddies the waters" of her constitutional role.
"I think it is daft," Professor Rodney Barker of the London School of Economics said.
"It will mean potentially the Queen will know things she is not supposed to know and hear things she is not supposed to hear.
"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."
The Queen stayed for around half an hour before leaving. She sat in the prime minister's chair, flanked by David Cameron to her right and foreign secretary William Hague to her left.
Her attendance as an "observer" was one of the final events marking the Queen's Diamond Jubilee year, in which she has visited all of the great institutions of state.
She posed for pictures with the prime minister on the steps of No 10 before entering on a red carpet. Conservative and Liberal Democrat ministers, who had all contributed to a gift for the head of state, lined up to greet her before the meeting began.
Bob Morris of UCL's Constitution Unit told politics.co.uk: "In a way you could say the visit of the Queen to the Cabinet shows in a way she no longer matters, because no harm's done."
The Queen is the first monarch to attend a Cabinet meeting since Queen Victoria's reign, Downing Street said yesterday.
That was disputed by historian Jane Ridley, however, who suggested the last time a monarch attended a peace-time Cabinet meeting was George III in 1781. King George VI attended meetings of the War Cabinet during the Second World War.
Earlier this year the Queen visited No 10 to attend a lunch with Tony Blair, Gordon Brown and John Major, also marking her Diamond Jubilee.
After today's Cabinet meeting the Queen exited with Hague, who took the monarch on a tour of the Foreign Office.
It was announced an area of around one-third of the British Antarctic Territory has been named 'Queen Elizabeth Land'.