Michael Fallon is to become the first Defence Secretary to visit the Falkland Islands in more than a decade.
He will head for the islands, where some 1,400 British service personnel are stationed, while the House of Commons is in recess this week.
But the trip is understood to have caused concern in the Foreign Office, with diplomats worried that it will inflame tensions with the new Argentine government and undermine attempts to improve relations.
Argentina’s previous president, Cristina Fernandez de Kirchner, stoked the war of words between London and Buenos Aires over the Falklands as part of an attempt to increase her popularity at home.
However her successor, Mauricio Macri, who took power last December, has adopted a less aggressive stance to the dispute over the islands’ future. He said recently he wanted to “restore” relations with the UK that had “been frozen in recent years, as a result of this conflict”.
A Whitehall source told the Mail on Sunday: “Michael has irritated the Foreign Office. They think it looks bellicose and will make it harder to do business with Buenos Aires.”
However a Ministry of Defence source added: “The Defence Secretary thinks it is important to show solidarity with the 1,400 British service personnel stationed there. Mr Hammond [the Foreign Secretary] does not have a problem with the visit.”
Last month, David Cameron warned Argentina’s new president that Britain will not negotiate over the sovereignty of the Falkland Islands. The Prime Minister told MPs a change of status would “never happen as long as I am in Downing Street”, unless it was backed by the local population, who in a 2013 referendum voted almost unanimously to remain British.