Britain is to step up efforts to fast-track European deportations of migrants from Iraq, Afghanistan and Eritrea.

At the European council in Brussels, Theresa May is expected to pledge an extra 40 staff over winter to help Greek border officials with asylum claims that Downing Street says are “likely to be deemed inadmissible”.

Greece is currently trialling a system to fast-track deportations with the aid of UK staff. “Rather than going into the asylum case flow they go into a faster process to return them [more quickly],” a Downing Street source said, adding that the 40 staff would have a number of specialists with experience of deportations. “It is exactly about that deterrent effect … The message is that you will be sent back.”

During the summit, at which migration will be one of three main topics, May is likely to express support for the continuation of the EU-Turkey deal that has seen Syrian refugees on the Greek islands returned to Turkey. In exchange Syrian asylum seekers in Turkey found a home in Europe, there has been a loosening of EU visa restrictions on Turks and Turkey has received around €6bn in aid.

A Downing Street official said the deal had radically reduced the numbers crossing the Aegean and the plan for new staff to speed up the process of returning failed asylum seekers would be a further deterrent to those attempting the perilous journey.

“It’s important to reduce the pull factor for migrants seeking a place in Europe, and getting better at differentiating between economic migrants and refugees needing protection,” the official said.

Downing Street said the prime minister was determined to use the UK’s influence to shape EU migration policy while Britain remains a member of the bloc. “There is a while yet while we will still be at the table [at the EU] and we think there are things we can be doing now to… reduce the numbers coming to Europe,” the official said.

“Once the UK has left the EU, there will be a number of issues we will want to co-operate on with other countries in Europe because of shared challenges that we face. We have a lot of expertise on returns.”

Syria will also be one of the main items on the EU leaders’ agenda. No 10 said May would push for tougher language from EU leaders on the crisis in Aleppo, but the focus will be on the need for humanitarian corridors and for information gathering on possible war crimes, rather than further sanctions against Russia.

“The PM will want to make sure the EU takes a tough stance on that given the appalling situation,” a Downing Street source said.

“The backdrop of this summit is Aleppo and the appalling crisis there and so our focus is how do we urge humanitarian access, and also to make clear that ... those who are carrying out ... atrocities in Aleppo should be held accountable.”

The prime minister is also expected to stress a need for closer EU ties with Egypt on the migration crisis. “They are a big source of unaccompanied minors,” a Downing Street official said. “We think it is right to be stepping up engagement with them.”

Before the summit, May will meet leaders from Latvia and Lithuania, as well as Martin Schulz, the president of the EU parliament. A No 10 source said the meeting had been at Schulz’s request but played down any suggestion there would be mention of his decision to run for the German parliament.

“This is focused on the role the European parliament will play in the Brexit negotiations,” the source said.

As well as the Syria crisis and EU migration, the leaders are expected to spend a working lunch discussing Ukraine and the recent Dutch referendum, which emphatically rejected a Ukraine-European Union treaty on closer political and economic ties. After lunch, they will attend a session on defence and security with Nato secretary general Jens Stoltenberg.

Downing Street said the prime minister was not concerned that the UK was not invited to the dinner of the EU27 and said no British observer would be present. “It’s their process to work out their approach. We’re preparing our side,” a No 10 source said.

May’s exclusion has been made possible because the EU council’s president, Donald Tusk, changed the format of EU summits, which have usually begun on Thursday afternoons with discussions until late into the night and conclusions announced on Friday mornings.

This council session will see meetings begin in the morning and conclude by early evening. May will give a press conference and return to London, while EU leaders meet for an informal dinner where the subject is expected to be the UK’s departure from the EU.