Copacabana Palace's official Belmond history says Princess Diana and the then Prince Charles checked in during their April 1991 Brazil visit and stayed in its Signature Presidential Suite. CNN Brasil and Folha independently corroborate the hotel, Rio dates and photographed pool visit.
Princess Diana stayed at Copacabana Palace in Rio de Janeiro during an official visit to Brazil in April 1991. The hotel’s own published history says Diana and the then Prince Charles checked in and identifies their accommodation as its Signature Presidential Suite. Independent Brazilian reporting confirms the hotel and places the Rio portion of the trip between April 24 and 26.
Source conclusion:
The cited sources support a stay at the named hotel.
