A spokesman for the EU Commission said the EU wants an “ambitious and fair partnership with the UK”, but a deal must be achieved by the end of October for it to be ratified in time.
On Monday, a No 10 spokesman said the Government is still confident a deal can be reached in September.
The comments come ahead of the latest round of negotiations between the UK and the EU, which are due to begin on Tuesday evening and continue until Friday.
Asked whether the EU is confident a deal can be achieved in September, an EU Commission spokesman said: “The important thing to note – and I would point you first of all back to what Michel Barnier himself said in London at the end of the last round of negotiations – that, first of all, we want a deal, we want to have an ambitious and fair partnership with the UK, and that we must come to an agreement in October at the latest.”
He added: “This week and over the coming weeks we will remain constructive, we will remain engaged and respectful with the UK negotiating team in order to reach a deal.”
Earlier this week, a No 10 spokesman said UK negotiators “will continue to plug the gaps where any differences remain”.
He added: “There are many issues that will be discussed during this week’s round, not least level playing field, fisheries, trading goods and services, amongst others.”
The trade talks resume amid fears of a deadlock, with both sides admitting after the last round of negotiations in London last month that they remained some way off a trade agreement.
After those talks, the EU’s chief negotiator Mr Barnier said a deal looked “at this point unlikely” given the UK position on fishing rights and competition rules.
Mr Barnier said the UK had not shown a “willingness to break the deadlock” on these issues.
He added there was a risk of no deal being achieved unless the UK changed its course on these topics which are “at the heart” of the EU’s trade interests, and that an agreement would be needed by October “at the latest” so it could be ratified before the post-Brexit transition period ends in December.
His UK counterpart David Frost agreed that “considerable gaps” remained in these areas, but argued that a deal was still possible.
The UK has ruled out extending the December deadline to reach a deal.