The UK will send dozens of officials to Greece as part of the effort to tackle the European Union's migration crisis.
The 40 staff are expected to speed up the processing of asylum claims from Iraqis, Afghans and Eritreans, many of whom will be sent back to the countries they came from.
The plan will be confirmed as Theresa May joins EU counterparts in Brussels for talks on the major diplomatic headaches facing the bloc including Syria and Ukraine.
US president-elect Donald Trump's criticism of Nato will also put the spotlight on the EU leaders' talks on defence cooperation and military spending.
The staff being sent to Greece in two phases from January follow 70 who have already been involved in efforts to address the migration crisis.
The mix of caseworkers, interpreters and people with expertise in returning people whose asylum claims have been rejected are aimed at sending out a message from the EU that economic migrants will be treated differently to refugees.
A Number 10 source said: "We have already been working quite closely with the Greeks to increase their capacity to deal with the numbers arriving in Greece and to look at how they step up returns.
"We are going to provide an extra 40 staff over the winter period to help with a trial that the Greeks have got under way to determine the admissibility of asylum claims from Iraqis, Afghan and Eritrean nationals."
Individuals from those countries are likely to be deemed inadmissible for asylum so their claims will go into a fast-track process "which enables you to return them quicker".
The Prime Minister is expected to tell the European Council summit that the continent needs to find a sustainable solution to the issue which responds to humanitarian need but is also aware of the way some of our communities are experiencing the "pressures of migration".
She is expected to warn that "failure to respond risks undermining our values and damaging the consent of our people to provide support and sanctuary to those who need it most," the source said.
As well as tackling the "pull" factor - by making it easier to send migrants back - leaders are also expected to look at measures to tackle the "push" factors driving people to risk their lives to reach the EU.
Mrs May will push for greater engagement with Egypt, a particular source of younger migrants, and stress her backing for the controversial EU-Turkey deal.
On Syria, the UK has pushed for a strong line to emerge from the summit following the "appalling" situation in Aleppo.
But there will be "differing views in the room about how strongly the EU should call out the action of the regime and its backers", following divisions among member states about the possibility of pushing for sanctions against Russia over Moscow's support for Bashar Assad's forces.
"We have been very clear that we think it is important to underline what the regime, Russia and other backers are doing and we want to focus particularly on the need for humanitarian aid," the source said.
On plans for closer defence cooperation, Downing Street wants to make sure that the EU's efforts complement the Nato alliance.
Mr Trump has previously criticised Nato and has complained that too much of the cost falls on the US, alarming some of the alliance's members in Europe.
Mrs May is expected to stress to EU leaders the importance of each country investing in its own military, pointing to the UK fulfilling its commitment of spending 2% of GDP on defence.