The Queen will attend Cabinet tomorrow for the first time in her reign, No 10 has announced.
Downing Street said the monarch would sit at the main table alongside her Conservative and Liberal Democrat ministers as an 'observer'.
It is believed this is the first time a British monarch has attended a Cabinet meeting since Queen Victoria's reign.
The Queen will be presented with a gift paid for out of Cabinet ministers' own pockets.
"This has been long-planned, as part of Diamond Jubilee celebrations," the prime minister's spokesperson said.
It is not clear whether she will be asked to make a short statement or even where she will sit. "I'm sure we can find room for one more," the spokesperson added.
Bob Morris, an expert on the monarchy at the UCL's Constitution Unit, said the Queen's attendance marked the last in a series of visits to the great institutions of state marking the Diamond Jubilee year.
"There's always this politeness - you always pretend they're the executive head of state, when they're not," he told politics.co.uk.
During Queen Victoria's reign the monarch struggled with liberal prime minister William Gladstone on some issues, prompting constitutional tensions. These have been manifestly absent under the present Queen.
Morris added: "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."