The Welsh-born princess is the widow of Prince Bertil, an uncle of Sweden's King Carl XVI Gustaf. Lilian and Bertil were married from 1976 until the prince's death in 1997.
The Swedish news agency TT said she was being cared for in her Stockholm home and does not feel well enough to attend the wedding of Crown Princess Victoria and Daniel Westling this month.
"Lilian is very tired and will not be coming to the wedding. She is old and frail," Elisabeth Palmstierna, marshal of the court, told the Swedish newspaper Aftonbladet.