Britain expressed concern on Wednesday at a decision by the South American trading block Mercosur to ban boats flying a Falkland Islands flag from docking at ports in Brazil, Argentina, Paraguay and Uruguay.
Mercosur announced the decision at a summit in the Uruguayan capital, Montevideo.
The ban will not include British-flagged civilian ships that supply the islands, although military vessels will be barred.
The move is the latest in a series of decisions taken by Mercosur members to show solidarity with Argentina, which claims sovereignty over the British-controlled islands it calls the Malvinas.
A British foreign office statement said: We are very concerned by this latest Argentine attempt to isolate the Falkland Islands people and damage their livelihoods, for which there is no justification.
Britain has controlled the islands since 1833 and fought a two-month war with Argentina over the territory in 1982, resulting in the deaths of 255 British and 649 Argentine soldiers.