UK officials increasingly recognise it would be a mistake to sever all defence, foreign policy and security links with bloc

The Ministry of Defence and Foreign Office are working on plans for how the UK could continue to cooperate with the European Union after Brexit, making it less likely that the cabinet will seek a fast-track divorce that severs all ties with the EU.

Including the areas of foreign policy and defence in any deal makes it more likely that talks will not be concluded in the two years required under article 50, meaning an interim deal will be required.

Some Eurosceptics, such as John Redwood, are pressing for a clean break by next summer, leaving only a basic trading relationship with the EU, and fear the talks could deliberately be made more complex in an effort to stall the process.

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Officials in key departments are said increasingly to recognise that it would be a mistake for the UK to sever all defence, foreign policy and security links with the EU, and that it would be easier to resolve those broader relationships after agreeing an interim deal.

It is also being argued that if the UK makes a clear offer to cooperate in these areas, it could oil the wheels in the more difficult negotiations over access to the EU single market.

The formal Brexit process set out by article 50 of the Lisbon treaty requires the UK first to agree the terms of its withdrawal and then to negotiate its future relationship. But the degree to which the EU will strictly enforce this sequencing, and the separation of the two processes, is unclear.

Sir Simon Fraser, a former permanent secretary at the Foreign Office, told MPs this week he believed the government wanted a continuing foreign and defence policy relationship with the EU despite Brexit.

He predicted the future relationship talks would cover not just economic issues but also cooperation on security, defence and foreign policy. “They will all be part of the future framework agreements between us and the EU,” he said.

He also said it was unrealistic to expect the talks on the UK’s future relationship with the EU, if it is to be wider than a simple trade relationship, to be completed inside two years.

That was “why we are confronted with the question of whether we are prepared just to go to article 50 and leave, and take the consequences, or whether we would like to negotiate an interim ongoing relationship until we get to the final state,” he said.

The desirability of the UK continuing to cooperate with the EU on defence and foreign policy issues is contested, but appears to be accepted by Downing Street. During the EU referendum campaign Theresa May based her support for remain partly on the need to retain security cooperation with the EU.

Although the government continues to rule out concepts such as an EU army or a defence headquarters, on the basis that they would duplicate the work of Nato, senior Conservatives say this should not mean ruling out future voluntary cooperation on EU defence and foreign policy.

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On Thursday Germany’s defence minister, Ursula von der Leyen, said of closer EU security cooperation: “The biggest resistance is coming from the British, and there we ask for fairness: whoever is leaving the EU should not in their last days block the caravan.”

But the British government recognises that it cannot veto EU defence cooperation once it leaves the bloc, and may need to think more strategically.

Boris Johnson, the foreign secretary, recently said any defence pact that undermined the power of Nato was a “bad idea”, but the UK should be prepared to cooperate militarily with the EU once it had left the bloc.

“If our friends want to go ahead with a new security architecture I don’t think, post-Brexit, we can reasonably stand in their way,” he said. “What we might suggest is that given that we are the biggest military player in the area, the only other nuclear power, it wouldn’t be a bad idea, if they do genuinely go ahead with such things, [to consider] a way in which Britain could be supportive, involved in the enterprise.”

Some diplomats argue that the unexpected election of Donald Trump as US president could make smaller EU states more eager to retain a strong defence relationship with the UK, the pre-eminent European military power.

One Tory MP said “The British cards are going to get stronger due to Trump. It is true if the 27 are determined to do themselves damage they can do it. But with the election of Trump, the security relationships have changed. Countries like the Baltic states will be demanding the EU behaves like grownups and stop saying we are going to punish the British. If we show goodwill over defence, and stop vetoing what they want to do, it is likely to change the atmosphere of the wider talks.”

Defence policy is not an area of EU activity that falls under the scope of EU law, and so relatively flexible ways of ensuring the continued participation of the UK in foreign and defence policy activities will be easier to develop than, say, partial membership of the single market.