The Duke of Cambridge, while speaking at an awards ceremony for members of the UK Foreign Office's Diplomatic Academy here, said Britain has always been "outward looking" with a "long and proud tradition of seeking out allies".
"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 and is central to your work. Right now, the big questions with which you wrestle -- in the UN, NATO, the Middle East and elsewhere -- are predicated on your commitment to working in partnership with others," the second-in-line to Britain's throne said.
The royal family traditionally stays neutral on political issues but Prince William's speech was widely interpreted as a timely comment to strengthen the camp which does not want the UK to break away from the economic bloc.
"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," the 33-year-old said.
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His speech today coincided with British Prime Minister David Cameron's crucial talks in Brussels for an EU reform agenda, which he hopes will strengthen his hand at campaigning for the UK to stay within the union once he does announce the date of an in-out referendum for the British public to have their say.
"I am not entering into the details of a plan B, because we don't have a plan B, we have a plan A. Britain will stay in the European Union as a constructive and active member of the Union," Cameron said during his ongoing Belgium visit.
Britain's ruling Conservative party MPs have already
entered into full-scale campaigning mode over the EU referendum, with two clear camps forming for and against staying in the EU.
Eurosceptics from the Grassroots Out campaign, United Kingdom Independence Party (UKIP) and Leave.EU will came together under the umbrella of 'GO Movement' to apply for official designation as the official leave campaign in the EU referendum.
The issue has deeply divided not only politicians but also prominent British celebrities.
While well-known actor Michael Caine recently spoke out in favour of Brexit, actress Emma Thompson made a firm stance in favour of staying in the EU at the BAFTA awards ceremony in London earlier this week.
The voices on both sides of the camp are expected to gather momentum as soon as a firm date for the in-out referendum is finally announced. The Opposition Labour party and the Scottish National Party (SNP) are all in favour of staying within the EU.