The Queen and Prince Philip have dropped in for lunch with David Cameron at Chequers.
It was only the monarch's third visit to the official country residence and was arranged by Cameron - who has weekly audiences at Buckingham Palace - so that he could "host her for a change", Number 10 said.
A menu of Scottish smoked salmon followed by lamb and then bread and butter pudding with ice cream was laid on for the royal couple and other invited guests.
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As well as Samantha Cameron, those around the table included former cabinet minister Lord Carrington - who was defence secretary when the Queen first visited the grace-and-favour mansion.
That was in 1970 when she joined visiting US president Richard Nixon for lunch at the invitation of then premier Edward Heath in what was seen as a special salute to the US first family.
It remains a very rare event at the Chilterns house gifted to the nation for use by prime ministers in 1921, with a 1996 visit as the guest of John Major believed by the Palace to be the only other.
Today's other guests were ex-head of MI5 Baroness Manningham-Buller, Lord Guthrie, the former head of the armed forces and writer William Shawcross, the Queen Mother's official biographer who chairs the Charity Commission and was accompanied by his wife, the designer and hotelier Olga Polizzi.
Royals have not always enjoyed such a welcome at Chequers - Lady Mary Grey was imprisoned there in a cell for two years in the 16th century.
It also houses a large collection of Oliver Cromwell memorabilia - his grandson having married the then owner of the house in 1715.
A list of visitors entertained by the Prime Minister was released by the Government in January and included senior politicians from both Coalition parties, Afghan president Hamid Karzai and Arts Council England chief Sir Peter
Bazalgette, the former boss of Big Brother producers Endemol.
But it included only those guests who had received official hospitality and Labour called on the PM to disclose those paid for out of his own pocket or by the Tory party.
After the meal, the Duke followed in the footsteps of the Queen on her last visit by planting an oak tree in the grounds.
Number 10 said Mr Cameron had been inspired to make the invitation after coming across archive photographs of the previous royal occasions.
The No 10 spokeswoman said: "The Prime Minister and the Queen see each other regularly. The Queen hosts the Prime Minister at the Palace once a week.
"The Prime Minister thought it would be a good opportunity to invite the Queen to have lunch with him, to host her for a change.
"When he has been at Chequers he found some old photos of previous visits from the Queen, including the visit she made during President Nixon's visit in 1970.
"That sparked the idea that it would be nice to host at Chequers again."Suggest a correction