Speaking at an event at the Foreign and Commonwealth Office, the Duke of Cambridge said:
"For centuries, Britain has been an outward looking nation. Hemmed in by sea, we have always sought to explore what is beyond the horizon. That sense of mission and curiosity is something that I know continues to drive our economy, our cultural and educational exports and our Armed Forces and Diplomatic Service. And wherever we go, we have a long and proud tradition of seeking out allies and partners.
"In an increasingly turbulent world, our ability to unite in common action with other nations is essential. It is the bedrock of our security and prosperity."
However, a Kensington Palace spokesperson said: "This speech was not about Europe."
Commenting on the Duke of Cambridge's speech, Liz Bilney, chief executive of Leave.EU, said:
"His Royal Highness talk to diplomats about maintaining partnerships as crucial to our country's interests is of course right, but it is taking a leap to suggest that this means we have to stay inside a failing institution like the European Union that does not always represent our interests or our views."
Prime Minister David Cameron is currently in Brussels ahead of the start of an EU summit on Thursday. Today he is believed to have met with a number of senior MEPs, as well as European Commission President Jean-Claude Juncker, to further discuss the negotiations.
Yesterday, banking giant HSBC, which announced it would be keeping its headquarters in the UK late on Sunday night, said that it would consider shipping staff out to Paris in the event of a Brexit.