Theresa May is meeting European Council President Donald Tusk for the first time since becoming UK prime minister later, with Brexit on the agenda.
In talks at Downing Street, Mrs May will say the UK will continue to play a full role in the EU until it leaves.
It comes as EU leaders, excluding the UK, prepare to meet next week to sketch out the bloc’s future after Brexit.
The PM has said the government will not reveal its hand too soon, amid pressure to set out its post-Brexit vision.
Mr Tusk – who oversaw the UK’s EU renegotiation prior to the in/out referendum – has said he wants to see a “velvet divorce” between the UK and EU, but he has also insisted that there will be no formal talks until the UK triggers the formal process for leaving under Article 50 of the Lisbon Treaty.
The meeting between Mrs May and Mr Tusk was being billed as an attempt for the two leaders to get to know each other ahead of those negotiations, BBC political correspondent Iain Watson said.
They will also discuss trade, migration, and Russian sanctions and the Ukraine.
The PM’s official spokeswoman said: “It will be an opportunity in part to talk about the process of leaving the European Union, how we see the upcoming months, but also to talk about the upcoming October European Council and some of the issues that we expect to be on the agenda for that.”
Donald Tusk is a businesslike pragmatist – hence his talk of turning the potentially fractious and intractable process of Brexit into a “velvet divorce”.
His visit to London is part of the process of preparing for the Bratislava gathering where the UK won’t be a voice around the conference table but will be very much an elephant in the room.
Neither he nor Theresa May is anywhere near deciding where their bottom lines will eventually settle on the issues of immigration and access to the single market – nor would either be ready to reveal their hand to the other if decisions had been taken.
But these talks could be important because they could help to set the tone for the opening phase of the more serious negotiations on Brexit whenever they might begin – and when they do it’s worth noting Mr Tusk will be representing the interests of the 27 states remaining in the EU, rather than the one that’s leaving.
The UK government has said it does not plan to kick-start the formal two-year EU exit process until the start of 2017 at the earliest, to give it time to prepare its negotiating position.
But it is facing calls at home and from abroad to clarify what it wants to achieve in the talks.
Mrs May has refused to give a “running commentary” on the Brexit process, and pledged to “think through the issues in a sober and considered way”.
“So we will not take decisions until we are ready. We will not reveal our hand prematurely and we will not provide a running commentary on every twist and turn of the negotiation,” she told MPs on Wednesday.
Meanwhile, EU leaders – apart from Mrs May, who has not been invited – will gather in Bratislava on 16 September to discuss the way forward for the bloc after the UK’s vote to leave the EU.
Speaking in Dublin on Wednesday, Mr Tusk said: “The Bratislava summit is not about Brexit per se – it is about bringing back political control of our common future.”
‘Right time’
After Thursday’s talks, the prime minister will chair a cabinet subcommittee on Britain’s exit from the EU which will focus on trade.
It comes after Australia and the UK began “preliminary discussions” about a new trade deal, with Australian trade minister Steven Ciobo predicting an agreement between the countries “when the time is right”.
But with the UK unable to sign deals while still in the EU, he said an agreement would not be able to happen until the UK left the EU in two-and-a-half years’ time.
Mrs May has said India, Mexico, South Korea and Singapore are also keen to remove trade barriers.
Brexit Secretary David Davis has predicted a “round of global trade deals” will be “fully negotiated” within 12 to 24 months, coming into force when the UK leaves the EU.
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