A deal on sharing top secret material after Brexit is essential if the UK is to continue defence, intelligence and cyber co-operation, officials say.
The UK is seeking a Security of Information agreement with the EU after it leaves based on arrangements the EU already has with the US and Canada.
It wants the accord to cover all EU agencies and allow the UK to “retain control” of information shared.
Security tensions over satellite access have flared up as EU-UK talks continue.
The UK has demanded £1bn back from the EU if the bloc carries through on its plan to exclude Britain from the Galileo satellite navigation system – and is also threatening to build a rival system.
Brussels has cited legal issues about sharing sensitive information with a non-member state – but the UK has said row could harm wider post-Brexit security co-operation and “risks being interpreted as a lack of trust in the United Kingdom”.
Arriving for a meeting of European finance ministers on Friday, Chancellor Philip Hammond said the UK wanted to remain a “core member” of Galileo but if that was not possible, it was committed to “going it alone”, potentially in partnership with the US and others.
“For national security strategic reasons, we need access to a system and we’ll ensure that we get it,” he said.
Theresa May has said the UK – which has one of the largest defence budgets of any EU country – remains unconditionally committed to the defence of Europe after it leaves the EU in March 2019.
The UK wants a separate defence and security treaty with the EU to enshrine existing co-operation.
But a senior EU official has attacked the UK’s “fantasy” approach to negotiations on this and other areas which they described as “let’s just keep everything we have now”.
In the latest salvo on security issues, the UK is to publish a paper which sets out its priorities with regard to the exchange and protection of classified and highly sensitive information.
The document, which was shown to EU negotiators earlier this week, makes clear an agreement is essential for the UK’s future co-operation in a range of areas, such as common EU security and defence operations.
Among “ongoing commitments” it lists are Operation Sophia, the Italian-led naval mission combating illegal migration in the Mediterranean which is being assisted by the Royal Navy, Operation Althea, a peacekeeping mission which upholds the 1995 Dayton Agreement in Bosnia-Herzegovina and EUTM Somalia, in which the UK and other EU countries train the Somali army.
“Without access to documents of this kind, the UK would not be able to manage the risk of deployments and would not be able to commit personnel or assets,” the document says.
The UK says it is in both sides’ interest for it to maintain its “significant” role within the EU’s Intelligence and Situation Centre, which assesses foreign policy and counter-terrorism threats to EU members and the bloc as a whole, including from Russia and other “hostile state actors”.
The paper also points out that the UK’s National Cyber Security Centre regularly shares intelligence with EU partners and helps attribute major attacks across the continent, citing the “NotPetya’ cyber assault on Ukrainian infrastructure in 2017, which affected businesses across Europe.
The UK says it hopes the EU will be able “go further” than in its existing accords with other countries, given how “broadly aligned” the two already are in terms of how different levels of information are treated.
But it says “existing networks” should be used to ensure no interruption in co-operation and that different security protocols should apply depending on how widely information is disseminated.
The EU has made it clear it wants certainty on key issues, such as the future of the Irish border and the UK’s proposed post-Brexit customs arrangements, by the time of a summit of European leaders next month.
Speaking in Brussels, Mr Hammond said unattributed negative comments by EU officials were “not particularly helpful”.
“There are obviously a wide range of views on both sides but everybody that I’ve engaged with has been very constructive and very keen to find a way to move forward,” he said.
“We’re very conscious of the ticking clock and the need to make significant progress for the June European Council.”
And, in a rare public comment, the top Brexit official in the civil service, Olly Robins, has defended the handling of negotiations.