Theresa May will be told that it is "not a given" that the EU will agree to move on to Brexit trade talks next month, as Brussels pushes for more information from her on key issues including the divorce bill the UK will pay to the EU.
The Prime Minister will hold talks with European Council president Donald Tusk and Ireland's prime minister Leo Varadkar as she steps up efforts to secure a breakthrough in the Brexit process.
The Prime Minister will meet Mr Tusk and the Taoiseach in the margins of a major European summit in Gothenburg on Friday.
European Union sources suggested that Mr Tusk will tell her that although internal preparations had begun on preparing for the second phase of Brexit talks, covering a transitional deal and the future relationship the UK will have with Brussels, it could not be taken for granted that leaders will agree to move on to formal talks.
Mrs May hopes December's European Council summit will see leaders give the green light to talks on a post-Brexit trade deal.
But a Brussels source said: "Mr Tusk will inform Mrs May that such a positive scenario is not a given, it will require more work and that time is short. And he will ask Mrs May how the UK plans to progress on the three key issues for phase one."
The three issues where "sufficient progress" must be made include the financial settlement, citizens' rights and the border between Northern Ireland and Ireland.
The Taoiseach is likely to emphasise that progress is possible in December but "only if all sides show sufficient political will", sources in Dublin indicated.
Mr Varadkar will use his meeting to emphasise the importance of upholding all aspects of the Good Friday Agreement, including the powersharing institutions, and will continue to urge the Northern Ireland parties to reach agreement.
The Prime Minister held talks with her Swedish counterpart Stefan Lofven on Thursday night.
A Downing Street spokesman said: "Both leaders said they wanted to see further positive discussions ahead of the December European Council."
If leaders do not agree to move to the second phase at the summit in Brussels on December 14-15, then it could mean no progress until the next scheduled European Council in March - adding to business uncertainty and increasing the potential for the UK to leave without a Brexit deal.
The Gothenburg social summit brings together political leaders and other key players to discuss a new European Pillar of Social Rights for workers' rights.
Mrs May will take part in a working session on fair employment and working conditions and is expected to highlight the findings of the Taylor report published in July which examined the treatment of workers in the so-called gig economy.
She will tell fellow leaders "how the the UK is addressing issues head-on in relation to the way employment practices are changing and the opportunities and challenges this presents and to point out how we've been consulting with business groups, trade unions and wider stakeholders".
The Government has committed to responding to the Taylor report by the end of the year.
TUC general secretary Frances O'Grady said: "Theresa May broke her promise to working people this week when she instructed her MPs to vote against protections for workers' rights after we leave the EU.
"The Prime Minister must now take steps to win back workers' trust by signing up to stronger rights for UK workers at the summit in Gothenburg.
"She must reassure working people that a level playing field for workers' rights will be written into the Brexit deal.
"Britain's workers must not be left to fall behind the rest of Europe when future improvements are made."