The talks resumed with the EU's chief Brexit negotiator, Michel Barnier, warning that Britain's security will suffer after it leaves the EU, in an apparent rebuke to UK Prime Minister Theresa May.
Writing in the French newspaper Le Monde, Barnier said Brexit "will have very practical consequences including on defence and security."
The article was published on Monday, as UK and EU teams converged on Brussels for up to four days of divorce talks. The talks were due to reconvene at 5pm Brussels time and are expected to conclude on Thursday.
In the article, Barnier said, "The British defence minister will no longer be able to sit at the council of defence ministers and London will leave the European Defence Agency and Europol."
The EU camp is reportedly infuriated at UK Brexit secretary David Davis's refusal to spell out how the UK's liabilities to the EU should be calculated.
Davis is expected to say on Monday that he wants to agree a deal which is in the best interests of both sides, and expects the talks to be constructive.
He will say, "For the UK, the week ahead is about driving forward the technical discussions across all the issues.
"We want to lock in the points where we agree, unpick the areas where we disagree, and make further progress on a range of issues. But in order to do that, we'll require flexibility and imagination from both sides."
Along with the UK divorce bill and citizens' rights, the situation of Northern Ireland will also be discussed this week, following the publication of a UK policy paper on that issue. One EU official criticised how the paper combines the issue "of the peace process and the issue of the future EU-UK relationship".