Sarah, Duchess of York will mourn the passing of Margaret Thatcher alongside the Queen and the Duke of Edinburgh.
It is a significant moment for the Duchess, whose relationship with the royal family has been strained since her divorce from Prince Andrew in 1996.
Now Sarah, who is mother to Princesses Beatrice and Eugenie, will attend the same event as her former in-laws when both parties pay their respects to Baroness Thatcher on Wednesday.
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The ex-royal, 53, was notably absent from Prince William's wedding to Kate Middleton in 2011. Her recent invitation is indicative of Sarah's friendship with Margaret Thatcher rather than a reconciliation with the Windsors.
"Sarah is honoured to have been invited and intends to go," a friend told the Telegraph. "She wants to pay her respects to Lady Thatcher."
When the former Prime Minister passed away last week, Sarah honoured her "steadfast" friend on Twitter. "Margaret Thatcher will be greatly missed," she wrote. She is part of all our history and was such a strong and consistent lady, steadfast to me in friendship."
Their bond developed after Baroness Thatcher attended Sarah's 1986 wedding to Prince Andrew.
"I was fortunate enough to get to know her from the time that she attended my wedding in 1986, and, subsequently, I shared numerous cups of tea with her, and loved to listen to her wisdom," she recently said.
Now the loss of that friend will join Sarah and the Queen in mourning.
According to reports, the last time the two women spent time together was in 2008 when the monarch invited her former daughter-in-law to join her at the Scottish country estate of Balmoral.
The most recent image of the pair dates back to 2004 when Sarah and her daughter Eugenie, seen in the first picture, joined the Queen at a Cartier International polo match held in Windsor.
On Wednesday, Sarah and the Queen will be joined by an array of figures from all walks of public life. London Mayor Boris Johnson and chef Marco Pierre White will attend alongside top military personnel and businessmen associated with the former PM's time in office.
Preperations are now underway for the ceremonial funeral, which will take place at St Paul's Cathedral.
More than 700 members of the Armed Forces carried out a full dress rehearsal in the early hours of Monday morning.
As dawn broke over the deserted London streets, pallbearers could be seen carrying a Union Jack-draped coffin into the cathedral.