Theresa May will meet outgoing US president Barack Obama later today and discuss the on-going threat posed by so-called Islamic State fighters.
Mr Obama has been on a farewell tour of Europe and will arrive in Berlin on Friday to hold a meeting with Mrs May, as well as the leaders of Germany, France, Italy and Spain.
His visit comes amid a backdrop of uncertainty and nervousness following the election of Donald Trump as Mr Obama's successor.
Mr Trump has been openly damning of Nato and sceptical of climate change.
However the main item on the agenda will be the growing concern of IS terrorists being forced out of Iraq and Syria, in the wake of coalition airstrikes.
A Downing Street spokesman said: "The issue that... will be raised here again is the issue of them [IS fighters] being dispersed around other countries in Africa and parts of the Middle East, not specifically back to the UK.
"As we close down Daesh's areas of operation in one part of the world we need to be mindful of the fact that they may regroup in other parts and how we set about tackling that.
"Obviously, the issue of Daesh fighters who've been in either Syria or Iraq returning to Europe is a matter of grave concern."
Brexit is not on the agenda, although Mrs May is expected to raise the situation with her European counterparts and German Chancellor Angela Merkel.
The spokesman added: "In the margins of the meeting almost certainly it will crop up in discussion. Brexit will be part of it, I'm sure".