Theresa May will set out the principles of the government’s negotiating position on the future rights of European Union citizens living in the UK at a dinner for EU leaders in Brussels on Thursday night – before leaving them to continue discussions over coffee without her.

The status of the 3 million citizens from elsewhere in the EU who have already made their lives in the UK has been a contentious issue since the immediate aftermath of the referendum.

May has repeatedly insisted she is not willing to guarantee their future without a reciprocal pledge about the future of British citizens living in other EU countries. Britain’s policy will be laid out in more detail in a government paper likely to be published on Monday.

The foreign secretary, Boris Johnson, said the UK would be making a “generous offer”, and hoped the EU would reciprocate.

But Brussels has made clear that it expects all EU citizens in the UK to retain their full rights – and for these to be underpinned by the European court of justice.

European leaders have no plans to discuss Brexit with May, preferring to leave Brexit negotiations in the hands of the EU’s chief negotiator, Michel Barnier. “President Tusk is not going to encourage a discussion on this point,” a senior EU diplomat said. “The European council of 28 is not a place for negotiations with the UK.”

It will be May’s first encounter with all EU leaders since the general election, at a summit that falls one year after the Brexit referendum.

On the eve of the referendum anniversary, EU diplomats say they remain in the dark about key aspects of the UK’s Brexit plans. “Any clarity about where they want to go is of course welcome,” another senior EU diplomat said. He said there was still misunderstanding in the UK about the EU’s intentions. The EU had no intention of punishing the British despite “a lot of spin” on the UK side, he added. “I think they are making life difficult for themselves.”

Brussels diplomats said their leaders would be keen for May’s update on the state of the British government, amid uncertainty about whether the Conservatives would have a deal with the DUP.

After this update, the British prime minister will leave early so the 27 other leaders can discuss Brexit without her over their after-dinner coffee and mints. “So she will have the chance to go and sleep,” quipped another diplomat.

The EU has made citizens’ rights a priority, along with the Brexit bill and avoiding a hard border between Northern Ireland and the Republic. Both the UK and the EU want an early agreement to protect the status of 3.5 million EU nationals in the UK and 1.2 million British people on the continent.

Despite this consensus, the issue is trickier than it looks. Outlining its official position last month, the EU said it needed detailed guarantees so citizens have the same rights to work, pensions, education and healthcare – so they can live “as if Brexit never happened”, in the words of one senior official.

The UK is expected to take a more minimalist approach. Another sticking point is the role of the European court of justice. The EU thinks the Luxembourg court should be the arbiter of disputes over citizens’ rights, an idea that is toxic to Tory backbenchers – though cabinet ministers have struck a markedly less combative tone on Brexit since the general election wiped out the Conservatives’ majority.

When negotiations opened on Monday, the Brexit secretary, David Davis, caved in to Brussels’ demands over the phasing of talks, with key issues such as the principles of the divorce bill due to be settled before a future trade deal can be discussed.

At the two-day summit, the prime minister will also announce a three-year package of measures to tackle the migration crisis, underlining the fact that the government expects to continue cooperating with the EU27 beyond Brexit on issues that it considers to be in Britain’s national interest.

The Department for International Development will allocate £75m to funding the voluntary repatriation of migrants passing through Africa en route to Europe, and to providing humanitarian assistance.

The aid will be targeted at the increasingly popular route from the Horn of Africa and west Africa through to countries including Niger, Egypt and particularly Libya.

The international development secretary, Priti Patel, said: “The UK has been at the forefront of responding to the migration crisis and our work to date has helped reduce total migrant numbers to Europe since 2015. But worryingly, more and more people are now using the incredibly dangerous central Mediterranean route.

“This new UK support will provide desperately needed aid and protection to tens of thousands of the world’s most vulnerable. But critically it will also make clear the massive risks involved at every stage of this route and provide alternatives, so those who change their minds can return home.

EU leaders are also expected to discuss defence and security, and May will call for more coordinated and stronger action on counter-terrorism – specifically on tackling online extremism.