LONDON — Add the future of a flagship EU military initiative to the uncertainty caused by Brexit.

Under current plans, Britain is due to provide most of the 1,500 soldiers for an EU battlegroup, a rapid-reaction military unit, from July 2019 — just months after it has left the bloc.

As part of their Brexit discussions, London and Brussels have to decide whether to go ahead with that six-month rotation, which could see British troops deployed on the orders of EU leaders.

The issue exposes bigger defense dilemmas for both sides. Although it has decided to leave the EU, Britain wants to remain an influential player in European security. And while the EU does not want post-Brexit Britain to enjoy the same status as a member country, the U.K. is a military heavyweight that can provide troops, hardware and expertise almost no other European nation can match.

How the two sides resolve this will reveal a lot about how much they are willing and able to cooperate on defense after Brexit.

Member countries take it in turns to provide the bulk of the troops for an EU battlegroup, a force established as part of a broader push to give the bloc more military clout. The battlegroup has yet to deploy to a crisis zone, although the EU has plans to make such a deployment easier by relieving the financial burden on nations that take part. But Britain is also heavily involved in other missions already underway as part of the EU's Common Security and Defense Policy that will have to be reviewed due to Brexit.

The U.K. government has made no decision yet about its 2019 rotation as lead nation in the battlegroup, two British officials said.

“The exact nature of our EU relationship and commitments post 2019 are to be determined as part of the Brexit negotiations. Until then we will continue to play a full and active part in EU discussions and we will remain committed to European peace security after we leave the EU in 2019,” said a spokesman for Britain's Ministry of Defence.

An EU official said Brussels and London had yet to discuss what would happen to Britain's role in the battlegroup after Brexit, which is due to be completed by March 2019. But the official noted that the legal basis for the battlegroup project was in EU law.

The U.K. government has made no decision yet about its 2019 rotation as lead nation in the battlegroup, two British officials said.

"It's reasonable to think that the United Kingdom, as a third country, will no longer be part of these battlegroups and therefore couldn't be in charge of one," the official said. But the official also noted that whatever is decided in the Brexit talks would not preclude Britain and the EU reaching a new agreement on security and defense cooperation.

Asked whether Britain would be able to lead the battlegroup in 2019, one senior French official said: "It poses a problem."

"We will have to see what we do about this. It's one of many defense-related subjects that we're going to have to examine. What do we do about information-sharing? There are a lot of issues to look at, one by one," added the official.

However, a defense expert in the German parliament said Berlin had a “great interest in the U.K. taking over leadership [of the battlegroup] in 2019.”

The official said such an arrangement would send a strong signal that the EU and the U.K. would continue to work together closely on defense after Brexit and could serve as a model for future cooperation with non-EU countries such as Tunisia, Israel or Turkey.

Decision-making

For the U.K., a key issue is whether, given its status as a major military player, it could secure more involvement in decision-making and control of missions than is normally offered to third countries.

If this proves impossible, it is unlikely Britain will continue to take part in EU military missions in future, according to U.K. officials. But if the two sides reach an agreement, the U.K. would have the option of contributing to EU operations, including providing battlegroups as part of a roster.

In addition to being able to provide the bulk of the troops for a battlegroup, Britain is one of only a handful of EU countries capable of providing a headquarters for the unit.

British ministers have already decided that the U.K. will honor its commitment to provide the headquarters for a battlegroup led by the Benelux countries in 2018. But British officials are skeptical that a solution to keep the U.K. involved in the second battlegroup can be found before the Brexit negotiations are wrapped up, ideally in the fall of next year, to allow the deal to be ratified.

Britain has a major role in other EU military missions. It currently leads the EU’s Atalanta counter-piracy mission off the coast of Somalia, commanding the operation from a base at Northwood, on the northern outskirts of London.

It also plays a significant part in Operation Sophia, which aims to counter migrant trafficking in the Mediterranean Sea and is under the command of the Italian military.

Brussels battle

Despite the resources and military expertise that Britain offers, however, some in Brussels see Brexit as a moment of liberation allowing the EU to forge much closer military and security links.

Britain has long worked to limit the scope of European defense cooperation, arguing that NATO should act as the cornerstone of European security, not the EU.

In the year since Britain voted to leave the EU, the bloc — led by Berlin and Paris — has pushed ahead with defense cooperation.

Earlier this year, the EU agreed to set up a Military Planning and Conduct Capability facility, seen by some countries as a first step toward a military HQ for the bloc. Brussels has also established a multi-billion-euro defense fund to finance joint military projects.

If it wants to maintain a leading role in the battlegroup and other EU defense initiatives, the U.K. will have to overcome considerable skepticism from decision-makers in Brussels.

In May, EU foreign affairs chief Federica Mogherini said Britain could still take part in missions, but could not enjoy the same status it had at the moment.

"Obviously, once you are not a member state you cannot take part in the decisions but you can take part [in the missions],” she said.

Britain was an "important military player but no way as important compared to the other 27 member states [combined]," she added.

Karl Pincherelle, a Socialist MEP on the European Parliament's defense committee, said that Britain would lack a clear legal basis for leading the battlegroup.

"Participation in battlegroups is determined on clear criteria: Only member states and those preparing to become members participate. It seems difficult for Britain to occupy this position," Pincherelle said.

He added: "I don't think a country that has decided to leave the process of European construction is well positioned to lead an entity that emerges from that construction process."

Janosch Delcker contributed reporting.