Security is quickly becoming a key front in the Brexit debate. UK Prime Minister David Cameron states bluntly that leaving the EU would threaten national security, and Downing Street on 23 February helped coordinate a public letter by eminent former defence officials arguing that EU membership was critical to deterring Russia, containing North Korea and combating Islamic State (IS). The last claim is timely: transnational terrorism has dominated European security concerns since the November 2015 attacks in Paris, and the Brexit debate is fuelled partly by fears that uncontrolled migration from the Middle East will import terrorist cells.

Looking at this issue from a threat perspective, Brexit is unlikely to have a major impact on the UK’s exposure to transnational terrorism (principally Islamist extremist terrorism). The main drivers of terrorism threat to the UK, particularly in terms of intent and capability of Islamist extremists, will persist regardless of EU membership.


The UK has been a top tier target for Islamist extremist groups – first al-Qaida and lately IS, as well as home-grown extremists – for more than fifteen years. Its extensive Middle East interests, muscular military deployments alongside the US and NATO since 2001 – Afghanistan, Iraq, Libya and now Syria (and Iraq again) – and close relations with Israel firmly include it in the so-called ‘crusader coalition’.

Jihadists also oppose the UK’s liberal political and cultural values, although – despite rhetorical pronouncements – these are not major motivating factors for terrorism. (Ironically, before the 7/7 bombings, these values increased the UK’s tolerance of Islamist extremists such as Abu Qatada and Abu Hamza.)

These factors are unlikely to change substantially regardless of the Brexit referendum. The UK’s close military cooperation with the US and NATO will persist as a matter of national strategy and security, helped by a gradual increase in defence spending over the next five years. The UK will maintain its extensive diplomatic and military footprint in the Middle East region, and remain engaged in a variety of counter-terrorism operations. Terrorist groups will continue to have high intent to target the UK regardless of its EU affiliation.


The shift in IS strategy towards high-impact transnational terrorism – a result of military setbacks in Syria and Iraq and reportedly lobbying by vengeful European jihadists – modestly elevates the terrorism threat in the UK. However, Brexit would be unlikely to significantly impact the capability of terrorist groups to carry out transnational attacks. The UK border is already less permeable than those in continental Europe, for both geographic and institutional reasons. Brexit would not make it significantly more or less difficult for foreign terrorist organisations to infiltrate the UK, or significantly change their ability to access weapons.

More importantly, the most likely terrorist threats in the UK stem from home-grown extremists, rather than foreign (even European) nationals. Of over thirty public Islamist extremist plots and attacks in the UK since 2001, nearly all were conducted by UK citizens or legal residents. The UK is actively targeted for recruitment by jihadist groups, and a sizeable contingent of UK nationals is also fighting alongside IS in Syria. Several of these have featured prominently in English-language IS propaganda. Brexit will not change the UK’s exposure to the unprecedented wave of jihadist incitement online.


The UK is unfortunately familiar with terrorism, both the decades-long campaign by the IRA and more recently Islamist extremism. As a result, UK security and intelligence services are highly capable and significantly mitigate the risk of a major attack, as well as many small-scale and unpredictable home-grown extremist attacks. 

That said, one of the most significant security concerns of Brexit is that it would disrupt vital intelligence cooperation with Europe – for example, via the fusion centre recently established by Europol. This seems unlikely: the UK and Europe would immediately craft new intelligence-sharing mechanisms in the interest of mutual security. In addition, the UK enjoys strong bilateral security and intelligence cooperation with key European allies such as France and Germany, and will continue to have a privileged relationship with the US as part of the Five Eyes. Brexit may well cause some institutional disruption (and fray bilateral relations), but counter-terrorism cooperation is likely to remain relatively insulated.


Get in touch

Can our experts help you?