Britain would "never" negotiate the sovereignty of the Falklands against its citizens' wishes, Prime Minister David Cameron said in a Christmas message to the islands claimed by Argentina.
"We will always maintain our commitment to you on any question of sovereignty. Your right to self-determination is the cornerstone of our policy," he said.
"We will never negotiate on the sovereignty of the Falkland Islands unless you, the Falkland Islanders, so wish. No democracy could ever do otherwise."
The Falklands, internally self-governing islands in the south Atlantic some 400 nautical miles from Argentina which claims them as part of its territory, have been held by Britain since 1833.
Britain and Argentina fought a brief but bloody war in 1982 over the islands, which are known as the Malvinas in the Spanish-speaking world.
On Friday, British foreign secretary William Hague asked Uruguay to let British ships dock at Uruguayan ports.
The request comes days after the South American trading bloc Mercosur -- which includes Brazil, Argentina and Uruguay -- agreed on Tuesday to close its ports to ships flying the Falkland flag.
In his message published by the BBC, Cameron criticised Argentina's "unjustified" attempts to disrupt shipping.
"We want to work with Argentina on those issues. But the Argentine government has continued to make statements which challenge your right to self-determination, and we can never accept that," he wrote.
Diplomatic friction between Argentina and Britain has intensified since 2010, when London authorised oil prospecting around the islands, which have a population of around 3,000.