Cameron gives the Queen a tour of Chequers after her first lunch at the PM's country retreat in two decades

The Queen and Prince Philip enjoyed lunch at Chequers today, for the first time in two decades.

David Cameron said he was 'delighted' to invite them to the Prime Minister’s country retreat after unearthing old photographs of a 1970 visit by US President Richard Nixon.

After a tour of the house, the rare occasion was marked by the Duke of Edinburgh planting an oak tree in the grounds.

Prime Ministers regularly use Chequers to host allies, world leaders and celebrities.

But the Queen has not been to the grace-and-favour home near Ellesborough in Buckinghamshire for 18 years, and has only ever been invited by Tory Prime Minister.

There were no trips under Gordon Brown, Tony Blair or Margaret Thatcher. The Queen last paid a visit when Sir John Major was prime minister in 1996.

After today's lunch Mr Cameron wrote on Twitter: 'Delighted to host Her Majesty the Queen and the Duke of Edinburgh for lunch at Chequers today.'

A Downing Street spokesman said: ‘The last time the Queen visited Chequers in 1996 she planted an oak tree in the grounds and the Prime Minister thought this would be a nice thing to do on this occasion.’

The menu for today’s lunch included Scottish smoked salmon for starter, a main course of lamb with potato and vegetables and bread and butter pudding and icecream for dessert, followed by cheese and biscuits.

According to chef Alan Lavender, who worked at Chequers from 1986 until 2010, Sir John served the Queen warm marinated salmon before presenting her with a jar of home-made Chequers chutney.

Her first visit was in October 1970 when she joined the then-PM Edward Heath and US president Richard Nixon.

She was photographed outside the red bricked Tudor mansion with the PM, Nixon and his wife Pat.

‘The PM and the Queen see each other regularly and the Queen hosts the PM at the palace once a week,’ the Downing Street spokesman added.

‘The PM thought this would be a good opportunity to have lunch with him, to host her for a change when he had been at Chequers he found some old photos of previous visits by the Queen including a visit with President Nixon in 1970 and thought it would be nice to host her again.’

At the time of the Nixon lunch several hundred people gathered at the main entrance of the estate, with a few holding anti-American placards calling for peace in Vietnam.

The guests dined on fillet of sole with lobster, smoked salmon, roast sirloin of beef and ice cream with mangoes, strawberries and peaches.

Chequers and its 1,000 acre estate were bequeathed to the nation in 1917 by Sir Arthur Lee, an MP and director-general of food production.

Lloyd George became its first prime ministerial occupant in 1921.

The Queen has dined at Downing Street numerous times. In July 2012, she and the Duke of Edinburgh joined Mr Cameron, Mr Brown, Mr Blair and Sir John for a lunch to mark her Diamond Jubilee.

Traditionally, the Queen invites the Prime Minister to make an annual visit to Balmoral, her private .residence in Aberdeenshire, Scotland, each September