Defence Secretary Philip Hammond was taken on board HMS Ocean by a landing craft, as it made its way up the Thames to Greenwich.
He addressed the ship's company in the hangar before taking a tour of the vessel.
– Defence Secretary Philip Hammond
"There are no specific threats to the Olympic Games but we're going to have very large numbers of foreign visitors in London."
"The world's eyes will be upon us and we want to make absolute sure that we do everything to maintain the security and safety of the Games.
"Ocean's presence on the Thames, I hope, will be a very great reassurance to people attending the Games."