The country will “lose influence” when it is ejected from the EU’s common security and defence policy (CSDP), the report by a House of Lords committee warns.
The UK “may be able to continue participating” in missions but “will not have the influence it currently enjoys in the development, planning and leadership” of those operations.
In particular, the report points to the EU’s successful mission to combat piracy in the Horn of Africa – led by the UK – and operations in Kosovo and the Western Balkans.
“CSDP missions and operations have contributed significantly to UK foreign policy and have benefited from the UK’s participation,” said Baroness Verma, the committee’s chair.
“Under the existing model for third country participation, the UK will lose influence over CSDP missions and operations”.
The best the UK could do was “seek to negotiate observer status in the EU’s planning and decision-making bodies”, the Lords EU External Affairs Sub-Committee says.
The verdict is the latest blow to Theresa May’s promise to pursue a new security treaty with the EU after Brexit that can maintain full cooperation, including through the CSDP.
Britain has already been accused of putting European security at risk because Brexit means leaving the European arrest warrant and Europol, the EU’s law enforcement agency.
In addition, the swapping of vital intelligence information is threatened by Ms May’s insistence that the UK will end oversight by the European Court of Justice (ECJ).
Meanwhile, the UK is locked in an increasingly bitter row with the EU over its exclusion from the Galileo satellite navigation system – prompting the government to threaten to curb intelligence sharing.
Senior figures in Brussels have also accused the UK of trying to blackmail the EU by using its strengths in intelligence and security as a bargaining chip in talks.
Now the Lords report has stressed the urgent need to clear up the confusion – with Brexit just 10 months away – calling for “detailed proposals” before the EU summit next month.
To do that, Britain needs to decide whether to use its military muscle to try to maintain some influence over joint security missions, the report says.
“The current model for third country participation offers a more limited role for the UK after Brexit than that envisaged by the government,” the report adds.
“The government will need to decide whether to use the UK’s significant military capabilities as leverage to modify the model for third country participation.
“The UK will also need to invest significant resources in Brussels and in member states’ capitals, to maintain influence from outside the structures of the EU.”
The key advantage of CSDP missions was their “lower-intensity crisis management”, pulling together military, political, diplomatic, economic and legal aspects, the peers found.
The UK’s contribution of personnel was “very limited” – just 2.3 per cent of the total – but it did provide planning guidance and “assets such as naval vessels and aircraft”.
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