Theresa May will reaffirm Britain's ongoing commitment to European security as she faces further pressure from EU leaders to spell out how she intends to settle the UK's Brexit "divorce bill".
The Prime Minister will hold fresh talks on Friday with European Council president Donald Tusk during a summit in Brussels between the EU and former Soviet bloc "partner" states.
With leaders from Armenia, Azerbaijan, Belarus, Georgia, Moldova, and Ukraine also due to take part, Mrs May will highlight Britain's continued support for the region in the face of the threat from a resurgent Russia.
She is expected to point to the £50 million the UK is providing this year to the region to support projects like tax reform in Moldova and de-mining in Ukraine, with a further £100 million over the next five years to counter "disinformation".
"From agriculture in Ukraine to the tech sector in Belarus, there is a huge amount of potential in the Eastern neighbourhood that we should nurture and develop," she is expected to say.
"But we must also be open-eyed to the actions of hostile states like Russia which threaten this potential and attempt to tear our collective strength apart.
"This summit highlights the crucial importance of the European countries working together to protect our shared values and ideals.
"The UK may be leaving the EU but we are not leaving Europe, and we are unconditionally committed to maintaining Europe's security."
Theresa May was warned by Mr Tusk the EU needs greater clarity over the UK position - including the "divorce bill" - by early December.
Since last week's EU gathering in Gothenburg, Sweden, the Cabinet has met to discuss what they would be prepared to pay to settle the bill, with ministers reportedly agreeing to double the sum originally put on the table by Mrs May to around £40 billion.
But it is thought she does not want to name a precise figure until she has a clear idea of what kind of trade deal is available with the remaining EU member states in the phase two negotiations.
Speaking ahead of Mrs May's talks with Mr Tusk, the Prime Minister's official spokesman played down suggestions it was a meeting to set out the divorce bill.
"I would not characterise it in that way," the spokesman said.
"We saw from his comments at the end of the press conference in Gothenburg that he (Mr Tusk) was hoping to meet the PM this week. They will be discussing progress towards the December council.
"There are a number of issues which I'm sure they will want to discuss - the financial settlement, that will be one of them, also, of course, Northern Ireland and citizens' rights."