Princess Anne is "not viewed as high level enough to represent the King" in Prince William's absence, royal sources have claimed.
The Prince of Wales, 41, is likely unable to replace King Charles during his Samoan tour later this year.
The monarch is thought to be adamant about travelling to Samoa for the October 21 Commonwealth Heads of Government Meeting (CHOGM).
Medical counsel has advised the King against long-term travel during his cancer treatment, but the 75-year-old is determined to go.
Prince William is Princess Anne's nephew and heir to the throne
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The Princess of Wales is also battling cancer, therefore, William is unlikely to fill in for his father as he cares for his wife.
Kate made her cancer diagnosis public in a moving video a few weeks after the King disclosed his illness.
The princess, 42, revealed that following her scheduled abdominal surgery in January, post-operative tests had revealed her diagnosis.
An insider told The Daily Express: "Attending the CHOGM is not something William can take on at this time due to Kate’s health, and although Anne and other working royals are doing a great job stepping in to plug the gaps at home, they are not viewed as high level enough to represent the sovereign as leader of the Commonwealth."
Princess Anne is not considered a 'high enough level' to represent King Charles abroad, sources have claimed
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Due to their roles not being senior enough, Princess Anne and Prince Edward will unlikely be permitted to attend.
The 76-year-old Queen will accompany the King on his October visit, should he be permitted to travel.
Discussing the tour, the source added: "King Charles's presence at both the Commonwealth meeting and in Australia is viewed as instrumental in preserving Commonwealth unity.
"Despite medical advice recommending against travelling long distances, particularly due to the time difference which can add to exhaustion, the King remains resolute in his decision to undertake the trip."
Prince Andrew visited Samoa on a royal tour in 1998
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Prince William is currently taking care of Kate and the three Wales children at Adelaide CottagePAThe five-day trip was previously confirmed for October 21–25, before Charles's diagnosis back in February.
Despite the King's cancer diagnosis, Buckingham Palace have said that "nothing is ruled in or out" with future engagements planned "subject to doctors advice".
Since battling cancer, the King has curtailed his public appearances.
On Easter Sunday at Windsor, King Charles led other members of the Royal Family at a church service.