Southampton-based Adonia and the American Star Princess were not allowed to berth at Ushuaia after docking at the Falkland Islands. It was on a tour of South America which left Southampton on 13 January.
A spokesman for the Foreign and Commonwealth Office said: "We are very concerned to hear the Adonia and Star Princess have been refused access to the port of Ushuaia.
"There can be no justification for interference in free and legitimate commerce.
"British diplomats in Argentina are urgently seeking to clarify the circumstances surrounding this incident, and we are in contact with the company concerned."
P&O says passengers will be refunded the cost of all shore excursions. Adonia is now sailing towards the next port of call, Punta Arenas; Chile."