The Prime Minister will tell MPs that the “ball is in the EU’s court” when she updates them on the progress of Brexit negotiations.
Theresa May will call for the European Union to be flexible in its approach and will promise the UK can get a deal that will prove the anti-Brexit “doomsayers” wrong.
It comes as the fifth round of Brexit talks between UK and EU negotiators are due to start today.
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Mrs May will say: “By approaching these negotiations in a constructive way – in a spirit of friendship and co-operation and with our sights firmly set on the future – I believe we can prove the doomsayers wrong.
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“I believe we can seize the opportunities of this defining moment in the history of our nation.
“A new, deep and special partnership between a sovereign United Kingdom and a strong and successful European Union is our ambition and our offer to our European friends.
“Achieving that partnership will require leadership and flexibility, not just from us but from our friends, the 27 nations of the EU.
“And as we look forward to the next stage, the ball is in their court. But I am optimistic we will receive a positive response.
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“Because what we are seeking is not just the best possible deal for us – but also the best possible deal for our European friends too.”
Mrs May’s recent speech in Florence was welcomed by EU leaders as constructive but that, and today’s words, may not be enough to move Brexit talks into the second phase.
European leaders will decide at a European Council summit on 19-20 October whether this level has been reached.
Progress is needed on three key issues – the divorce bill, citizens’ rights and the border between the UK and Republic of Ireland – before talks can move on to post-Brexit relationship issues, such as UK-EU trade.
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But European Commission president Jean-Claude Juncker said last month that it would take “miracles” for enough progress to have been made before the leaders’ meeting.
Mr Juncker had appeared much less optimistic over the advancement of Brexit talks than EU chief negotiator Michel Barnier, who completed the latest round of discussions a few days earlier.
Despite warning it could take “several months” for the key divorce issues to be decided, Mr Barnier hailed a “new dynamic” to negotiations following Mrs May’s Florence speech.