BRUSSELS — The European Union security chief Julian King has called for the “closest possible cooperation” on defense and security issues after the U.K. leaves the 28-member bloc.

“On some issues there will be winners and losers, but there is a mutual, shared self-interest when it comes to security and defense,” King said.

Despite the failure of last week’s EU summit in Salzburg, Austria, to back British Prime Minister Theresa May’s latest Brexit proposals, King remains “optimistic” the U.K. and EU could continue to work together on security and defense.

Some have questioned the effectiveness of European defense and security after the departure of the U.K. Britain is the second-largest net contributor to the EU, and its exit will result in an income shortfall of about €84 billion (U.S. $99 billion) for the EU’s next spending period from 2020.

Another problem, according to U.S. President Donald Trump, is the continued “unwillingness of some member states to contribute more” to NATO.

Speaking at a security debate in Brussels, King highlighted cyber and the ongoing threat from terrorism as key areas where the two sides must cooperate post Brexit, which will occur at the end of March 2019.

“Of course, there are still a few things still to resolve between now and March, and the economic side will be tough. But we need the closest possible cooperation in tackling the security challenges we both face, and I am optimistic we can do this,” he said.

“Those people who are trying to harm us do not make a distinction between member states. We are facing shared threats, which are best tackled if we act together. This is true today, and it will be true after March 2019. It is this shared self-interest that I believe will drive cooperation on the security side,” he added.

There was “no dispute” on the need to support member states in the security and defense field, according to King, but rather the challenge is finding ways to strengthen such collaboration.

Julian King, the head of the EU's security unit, addresses the media at the European Commission in Brussels on Sept. 7, 2017. (Emmanuel Dunand/AFP via Getty Images)
Julian King, the head of the EU's security unit, addresses the media at the European Commission in Brussels on Sept. 7, 2017. (Emmanuel Dunand/AFP via Getty Images)

He praised recent EU investment in new security and defense initiatives, such as the Permanent Structured Cooperation and the Coordinated Annual Review on Defence, which, he pointed out, involve “tens of billions of euros.”

While the EU was “doing OK” in tackling the twin threats posed by cyberwarfare and terrorism, he conceded that there is “an enormous amount still to do.”

One example, he said, involves addressing artificial intelligence. This could be a force for both good and bad, he suggested, but the EU has been “slow” in responding to the challenges posed by AI.

King’s comments were echoed by Jaap de Hoop Scheffer, NATO secretary general from 2004-2009, who said he also hopes that, post Brexit, the EU and U.K. will enjoy the “closest possible cooperation” on defense and security issues. “Yes, we all want the EU to take more responsibility in the defense sphere, but you have to ask: ‘What is European defense without the U.K.?’ ”

Both were speaking during “A Brave New World,” a debate organized by Friends of Europe, a leading Brussels-based think tank.

Elsewhere, a leading U.K.-based academic warned that the “Salzburg impasse” puts EU-U.K. security cooperation at risk. Last week’s summit of EU leaders in Salzburg ended acrimoniously with the EU saying May’s trade proposals “would not work.” This has led May to demand “more respect” from the EU side.

Dimitrios Giannoulopoulos, a professor at the University of London, said that “reaching a new security agreement independently of the main Brexit negotiations will be easier said than done.”

“The threat of a ‘no deal’ Brexit would seriously disrupt U.K. and EU capabilities in the fight against terrorism and organized crime, at a time when the EU is committed to stepping up its efforts to improve judicial and police cooperation in Europe," he said.

Speaking separately, Gordon Sondland, the new U.S. ambassador to the EU, has pledged to work with the EU “honestly and constructively to address the global security threats that seek to destroy our shared history, values and culture.”

“Whether defeating the Islamic State, countering North Korea’s belligerency or ensuring energy supplies will never be used for political coercion, we will stand together," the diplomat said. "There are a wealth of issues we can tackle together. From malign Russian activity (ranging from disinformation campaigns to invasion and occupation of sovereign nations), to data privacy, to Iran — yes, even Iran — we work best when we work in tandem.”