It appears she had made an unsuccessful attempt to hand over a letter - presumably addressed to her sister Meghan.
Samantha first publicly slammed Meghan after her romance with Harry became public knowledge calling her a "social climber" who is "not fit to be a royal".
Speaking to United Kingdom tabloid the Mirror, a source said the latest development "spoke volumes".
A source close to Samantha told The Mirror, "She fully hoped they would let her in but she was turned away".
Was Meghan Markle's older half-sister Samantha really refused entrance when she showed up uninvited at Kensington Palace?
Samantha, who is confined to a wheelchair due to multiple sclerosis, was accompanied by her partner.
"She went to the gates and tried to reason with them", a source told The Sun.
Meghan, Duchess of Sussex makes an official visit to the Joff Youth Centre in Peacehaven, Sussex on October 3, 2018 in Peacehaven, United Kingdom. "It was a sad sight". "She's mindful that she's representing the Queen and the royal family and doesn't want to put a foot wrong".
"I think everybody was hurt at not being included or invited to the wedding", she concluded.
According to The Mirror, Samantha flew into the United Kingdom two weeks ago in an attempt to "make peace" with her younger sibling - this, after slating her actress sister on countless occasions on social media.
Grant arrived in London more than a week ago with the intention of seeing her estranged sister, according to an interview she gave on the U.K.'s Channel 5 show with Jeremy Vine.
"I felt it could've all been nipped in the bud had everyone been included and we all agreed to move forward with positive resolve".
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