Michael Clarke, director-general of the Royal United Services Institute, said the concerns being raised about the lack of an aircraft carrier were "fairly meaningless".
Admiral Sir John Woodward, who led the taskforce to recover the islands in 1982, earlier told The Times that while Britain is without an aircraft carrier it would not be able to repeat the successful mission of 30 years ago.
However, Clarke dismissed fears of an invasion. Speaking to BBC Radio 4's World at One, he said: "The defence of the Falklands now - the Argentineans have got absolutely nothing they could use to invade with."
"I challenge anybody to give me a sensible invasion plan."
He said the UK has "more combat power but on fewer platforms" than it previously had and insisted the RAF could be there "within 18 hours".
Earlier on Monday, the naval officer responsible for co-ordinating the torpedo attack which sank the General Belgrano, creating a turning point in the Falklands War, said today that he had no regrets.
Vice admiral Sir Tim McClement was second-in-command of the submarine HMS Conqueror which fired the torpedoes at the Argentinean warship, causing the loss of 323 lives.
Margaret Thatcher was heavily criticised for the move as the ship was outside an exclusion zone and was heading away from the Falklands.
But Sir Tim, from Wickham, Hampshire, whose task was to organise the attack on board the hunter-killer submarine, defended the strike.
The 60-year-old told the Portsmouth News: "There is no doubt in my mind that sinking the Belgrano was absolutely the right thing to do - firstly for survival in case the pincer movement worked against our carriers and secondly it demonstrated intent to the Argentinians."
He added that following the sinking, the Argentinean navy was deterred from participating in the conflict.
And speaking of the Argentinian loss of life, he said: "They started it, so all lives lost are the Argentinean government's fault.
"In war tough decisions have to be made and people die."
On Monday, Defence Secretary Philip Hammond rejected claims Britain would be unable to defend the Falklands against a fresh Argentinean assault as both countries mark the 30-year anniversary of the conflict in the South Atlantic.