The Queen is to attend a cabinet meeting on Tuesday as an observer, Downing Street has announced.
It is believed to be the first time a monarch has attended cabinet since Queen Victoria's reign.
The cabinet, which meets weekly, will present her with a gift which they have bought to commemorate her Diamond Jubilee year.
She is expected to sit beside the prime minister during part of the regular meeting of senior ministers.
While the Queen is head of state, her involvement in day-to-day political decisions is largely formal.
The prime minister visits her regularly for an audience where he updates her on events, while she is also expected to rubber-stamp ministerial decisions at meetings of the Privy Council.
The Queen plays a central ceremonial role in the state opening of Parliament, when she travels by ornate horse-drawn coach to the House of Lords to read out a speech prepared by ministers unveiling details of their legislative plans.
She also retains the power to appoint the prime minister.
BBC political correspondent Chris Mason said: "She's well used to hosting a weekly audience with her prime ministers, David Cameron being her 12th, but has never headed into Downing Street, sat there in the cabinet room and heard her various ministers discuss whatever happens to be in the government's in-tray that week.
"Tomorrow she will do that for the first time in her 60 years on the throne."
The prime minister's spokesman said the Queen would spend about 30 minutes at the meeting, sitting between the PM and the foreign secretary.
The spokesman said he imagined she would speak, despite being described as an observer.
But Rodney Barker, professor of government at the London School of Economics, said the plan was "daft", because "it will mean potentially the Queen will know things she is not supposed to know and hear things she is not supposed to hear".