Downing Street has rejected Argentine claims that Britain is creating a risk to international security by "militarising" the long-running dispute over the Falkland Islands.

President Cristina Fernandez de Kirchner has said Argentina will make a formal complaint to the United Nations over the UK's decision to send one of its most modern warships and the Duke of Cambridge to the South Atlantic.

But No 10 said the despatch of destroyer HMS Dauntless to the Falklands to replace the frigate HMS Montrose did not amount to an escalation of military activity.

"We are not militarising the South Atlantic," said a Downing Street spokeswoman. "Our defensive posture in the Falklands remains the same."

The spokeswoman confirmed that the Government has contingency plans in case of aggressive actions towards the self-governing British overseas territory, but stressed that the contingency plans have been in place for some time, and are not a response to the recent spike in tension between London and Buenos Aires as the 30th anniversary of the war for the islands approaches.

Ms Kirchner's comments are the latest in a series of very public displays of anger from Buenos Aires over development surrounding the disputed archipelago.

Last week William began a six-week posting in the region in his role as an RAF search and rescue pilot, his deployment coming after the Government confirmed it was sending Dauntless, one of its newest Type 45 destroyers, to the South Atlantic.

The warship is due to set sail for the region on her maiden mission in the coming months to replace the Montrose in a handover of responsibility described as "entirely routine" by Foreign Secretary William Hague.

It has also been reported that the Royal Navy is sending a nuclear submarine to the region to protect the islands from possible Argentinian military action. This has not been confirmed by the Ministry of Defence.

Speaking to reporters at a daily Westminster briefing, the Downing Street spokeswoman said it was a matter for Argentina what issues it wished to raise at the UN. She said: "The people of the Falklands choose to be British. Their right to self-determination is a principle enshrined in the UN charter. The Falklands are already discussed by a UN committee and it is up to the Argentines to decide whether they are going to raise it at the UN."