Theresa May should refrain from using Britain's role as the leading west European military and intelligence power as a bargaining chip in Brexit negotiations, a respected defence think-tank has said.
It follows suggestions that the Prime Minister could use the UK's critical role in the continent's security as a trump card in divorce talks to secure a favourable post-Brexit trade deal, potentially including access to the single market.
The Royal United Services Institute (RUSI) cautioned Mrs May against such an approach, warning that both allies and potential enemies would see it as a more deep-seated move towards isolationism than previously feared.
It would also call into question the UK's reliability in contributing to collective defence and, therefore, its commitment to NATO.
In a briefing on the UK's post-Brexit foreign and security policy, RUSI Deputy Director-General Professor Malcolm Chalmers wrote:
"Where the UK's 'security surplus' can be useful is in making it clear to its EU negotiating peers - especially in security and foreign affairs - that there is more to the relationship with the UK than haggling over tariffs, migration rules and budget contributions.
"After Brexit, the UK and the EU will continue to share fundamental interests and values, and the UK will continue to be a reliable defence and security partner.
"In considering how to handle the negotiations as they reach moments of crisis, any desire to punish the UK in order to deter further defections from the EU should be properly considered in the context of the broader relationship with the UK."
He also warned that Britain could lose its senior position in NATO's command structure. The country has held the position of Deputy Supreme Allied Commander since 1951 but this may have to be transferred to a member of the EU after Brexit.
The think-tank said a solution might be the creation of a second equivalent position within NATO or the UK swapping its role for another senior one.
The impact of a Donald Trump presidency in the United States also has implications for post-Brexit Britain.
The briefing said that the UK must resist any growing feeling that it has to stand "shoulder to shoulder" with the US in future military conflicts, highlighting the highly criticised 2003 Iraq invasion as a cautionary tale.
Professor Chalmers said:
"The UK's departure from the EU is likely to deepen the recent trend towards a security policy focused on national interest.
"Trump's election - on a platform of 'America First' - could further encourage this trend, throwing further doubt on whether the post-1945 Western institutional order can now survive."