Carrie Symonds will accompany her boyfriend Boris Johnson when he stays with the Queen at Balmoral later this summer.
The Prime Minister’s girlfriend, 31, will attend the traditional weekend at the monarch’s Scottish estate, sources told the PA news agency.
Every year the Queen spends the summer at Balmoral Castle, Aberdeenshire, where she is joined by other members of the royal family – and traditionally hosts the prime minister for a weekend in September.
It is thought that Ms Symonds will be the first unmarried partner of a sitting PM to stay at Balmoral.
Former prime minister David Cameron once said there was not much “chillaxing” – chilling out and relaxing – at the castle, with the royals spending their time on outdoor pursuits.
But Ms Symonds, who last week gave a speech on protecting the environment, may find herself at odds with the royals, whose pastimes famously include blood sports.
The public relations expert, who lives in Downing Street with Mr Johnson, 55, resigned as director of communications for the Conservatives last year and is now a senior adviser at Oceana, a US-based environmental campaign group.
Baroness Thatcher reportedly found the stay at Balmoral painful, with an observer writing that a “weekend in the country with aristocrats who enjoy riding, shooting, sports and games is Thatcher’s idea of torture”.
However Cherie Blair revealed in her autobiography how her son Leo was conceived at Balmoral when she left her contraception at home out of embarrassment during her annual weekend stay there with then-PM husband Tony.
At royal residences, servants meticulously unpack luggage for guests, and Mrs Blair said: “In 1998 – I had been extremely disconcerted to discover that everything of mine had been unpacked.
“Not only my clothes, but the entire contents of my distinctly ancient toilet bag with its range of unmentionables.
“This year I had been a little more circumspect, and had not packed my contraceptive equipment out of sheer embarrassment.
“As usual up there, it had been bitterly cold, and what with one thing and another…”