THERESA May has said she is "ambitious and positive" for Britain's negotiations with the EU but there is still "some way to go" in the talks.

The Prime Minister was speaking after leaders of the remaining 27 EU member states gave the green light for preparations to begin for the second phase of Brexit talks, dealing with trade.

The move paves the way for the possible start of formal talks on the future EU/UK trade relationship in December.

The European Council in Brussels decided that insufficient progress had so far been made in divorce talks to move on to trade discussions now, as Britain had hoped.

But Council president Donald Tusk said in a tweet: "Brexit conclusions adopted. Leaders green-light internal EU27 preparations for 2nd phase."

At a press conference after her participation in the summit concluded - and while the remaining 27 leaders continued their discussions - Mrs May said: "I am ambitious and positive for Britain's future and for these negotiations.

"But I know we still have some way to go. Both sides have approached these talks with professionalism and a constructive spirit and we should recognise what has been achieved to date."

The agreement among the 27 leaders to begin preparatory work on trade talks means scoping discussions are now expected to begin in Brussels in time for the next scheduled Council summit on December 14-15.

Until then, formal discussions with the UK will continue to focus on the "divorce" issues of citizens' rights, the Irish border and Britain's financial settlement.

EU leaders made clear on Friday morning that they are looking for more clarity from the UK about the size of its divorce bill before giving the go-ahead for trade talks to get under way.

But Mrs May repeatedly dodged attempts by reporters to get her to reveal the sum she was prepared to hand over to Brussels, insisting that the financial settlement could only be finalised as part of the overall Brexit deal.

She said officials were going through the issues "line by line" to decide what was owed and added: "The full and final settlement will come as part of the final agreement that we are getting in relation to the future partnership and I think that's absolutely right, I think that can only be done in that particular context."

Earlier, German Chancellor Angela Merkel gave an upbeat assessment of the talks, saying they were making progress "step by step".

"I have no doubt that if we are all in clear minds ... We are going to achieve a good outcome," she said. "As far as I am concerned, I don't hear any reason to believe that we are not going to be successful."

Mrs Merkel said she was "highly motivated" to work on a new mandate for chief Brexit negotiator Michel Barnier which might permit trade talks to begin in December, but warned that the second stage of talks would be "more complicated than the first".

On the divorce issues dealt with in the first stage, there was "by and large general agreement" on the future status of the Irish border and "headway" was being made on expats' rights after Brexit, but Mrs May made no specific new offer on Britain's financial settlement, she said.

Dutch Prime Minister Mark Rutte said the EU27 need "more meat on the bone" of Britain's exit payment, following Mrs May's promise in a speech in Florence last month that the UK would honour financial commitments made as an EU member.

The offer made by Mrs May in Italy is believed to amount to around 20 billion euros (£18 billion), while Brussels is understood to be seeking something closer to 60 billion euros.

Mrs May said: "As I understand it, following the discussion that has taken place this morning, we will see the EU considering their response to the Florence speech in terms of the sort of vision that they have for the partnership that we can have for the future."

Pressed on reports that the Government was planning an "upbeat" assessment of a no-deal scenario, the PM said: "I have been clear that we have been working for a good deal and I am optimistic about getting that.

"But, as I have said, and others have said consistently, it would be irresponsible for the British Government not to look across at the changes that will be necessary regardless of the eventuality.

"And, indeed, some of the changes that will be necessary ... in the event of a no deal would be the same as changes that would be made in relation to us achieving a deal.

"And, when the Cabinet comes to look at this it will be looking across the board at what work is being done across Government departments for all eventualities, and that includes for getting the good deal, the positive deal, that I am confident that we can achieve, and we have seen from European leaders here, believe is achievable as well."

Mrs May said the two sides were within "touching distance" on a deal regarding citizens' rights.