The Prime Minister’s officials insisted there were still “big issues” to resolve and warned that “movement” was required from Brussels to break the current deadlock in the negotiations.
Whitehall insiders also sought to dampen hopes of an imminent deal, accusing the EU of trying to build up expectations in an attempt to put pressure on the British negotiating team.
Monday’s caution from Downing Street followed a series of optimistic remarks from senior figures in the bloc in recent days.
Rumours about an imminent Brexit deal have been circulating in Brussels ahead of a summit in the city next week where Britain’s departure from the bloc will be a key item on the agenda.
Mrs May’s official spokesman said: “We have always said that we are working hard for a deal this autumn and that continues at pace.
“It’s worth pointing out that there is a difference between people talking optimistically about a deal and deal including both a withdrawal agreement and a future framework actually being agreed.
“There remain big issues to work through, and as the Prime Minister has said this will require movement on the EU’s side.
“There can be no withdrawal agreement without a precise future framework.”
The spokesman also refused to confirm reports that EU Exit Secretary Dominic Raab will hold a fresh round of head-to-head talks with chief EU Brexit negotiator Michel Barnier in Brussels this Thursday.
He said: ”There are technical discussions taking place in Brussels this week – those continue.”
His insistence that Britain wanted a “precise” framework for the future relationship between the UK and the EU followed expectations that the bloc could offer a sketchy draft agreement leaving many details to be concluded over several years.
Such a move would almost certainly enrage Brexit-backing Tory MPs who fear Eurocrats are plotting to keep Britain effectively tied to the bloc indefinitely.
The Prime Minister’s spokesman said: “We have said that when MPs take part in a meaningful vote, they must be able to do so on an informed basis.
“We are working hard for a deal this autumn – that is what we are working towards.”
Another Whitehall source said: “Brussels are trying to play up expectations because they have every interest in doing so. The EU always does this in every negotiation it is involved with.
“They want to raise expectations ahead of the summit in order to put pressure on us. There is still a lot to discuss.”
Speculation about a breakthrough was fuelled at the weekend when Ireland’s deputy prime minister Simon Coveney claimed the deal was “90 percent there”.
His assessment was more optimistic than the 75-80 percent figure previously quoted by Mr Barnier.
EU Council president Donald Tusk last week said the bloc was trying for an accord in October and he believed it could be agreed by the end of the year.
And European Commission president Jean-Claude Juncker said on Saturday that he believed the “rapprochement potential” between the two sides had increased in recent days.
“My assumption is that we will reach an accord which will achieve the conditions of the withdrawal treaty,” Mr Juncker told the Austrian media.
“But it cannot be foreseen whether we will finish in October. If not, we’ll do it in November.”
Reports suggest that Mrs May hopes to break the deadlock over the Irish border by keeping the EU’s present customs arrangements beyond when the transition period is due to end in December 2020.
But the Prime Minister’s spokesman today refused to discuss the suggestion.
Senior Tory MP Steve Baker, the deputy chairman of the European Research Group of Eurosceptic backbenchers, tonight raised concerns about the progress of the negotiations.
He feared some officials in Whitehall may be pressing for the EU to stay in the EU’s single market and customs union.
“They may be preparing the ground to re-join the EU later,” the former Brexit minister said.
“We must actually leave, not leave in name only or ask the question again.”
The European Research Group released a series of internet videos seeking to warn voters of the risk of a possible capitulation to Brussels in the Brexit negotiations.