Maria Gallastegui has promised Prince William that she will cover up her anti-war signs on the day of his wedding.
The 52-year-old grandmother has been protesting at Parliament Square, London, for nine years.
Five years ago she gave up her job as a coach driver and sold her house to live on the site in a tent fulltime.
The wedding procession will pass by her "peaceful protest" and other demonstrations, which have been there since 2001, on the way to and from Westminster Abbey.
Britain's Prime Minister, the Home Secretary and the Mayor of London have all vowed to move the groups but despite numerous legal attempts have been unable to do so.
Wearing a T-shirt with the words "Freedom for Palestine", Ms Gallastegui told the Herald she had written to the Queen, Prince Charles and Prince William respectively to offer a truce for the big day.
She received a letter back, on St James' Palace stationery, which read: "Prince William acknowledges your intention to cover the signs on April 29" and that "careful note has been taken on the points you make". The Prince "appreciates your letter".
It is signed by Claudia Holloway who, as head of correspondence for the palace, is in charge of replying to the thousands of letters the Prince of Wales and his sons receive a year.
Ms Gallastegui also received a letter on Clarence House letterhead from Prince Charles with the same message.
She has not heard from the Queen.
"I appreciate their reply. They took on board what I said. It's very good to have communication."
Two makeshift police boxes will be covered, one with the first words of a Wilfred Owen poem: "Dulce et decorum est pro patria mori".
It was written during World War I and translates to "How sweet and fitting it is to die for one's country".
But Ms Gallastegui has promised to ramp up her efforts with more signs the day after the ceremony.
Meanwhile, another camp, led by veteran anti-Iraq war protester Brian Haw, said it had no plans to cover its signs.
Most protesters have permission to stay on the pavement under a clause in the Serious and Organised Crime Act 2005.