Negotiations between the European Union and the UK on a post-Brexit deal are due to resume in Brussels.
The two sides are still far apart on key issues including fishing rights and competition rules.
The trade negotiations are scheduled to continue until 2 October - less than a fortnight before an EU summit.
The EU has reiterated that negotiators must seal any deal "in October at the latest" to allow time for its ratification by member states.
A spokesman for the EU Commission said the EU wants an "ambitious and fair partnership with the UK".
On Monday, a Downing Street spokesman said the UK government is still confident a deal can be reached in September.
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, Michel 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.
His UK counterpart David Frost agreed that "considerable gaps" remained in these areas, but argued that a deal was still possible.
The EU said Britain's proximity and past membership mean it must abide more closely to EU standards than other nations if it wants open market access.
The British government countered that it should get the same treatment the EU has given other independent states that signed up to trade deals.
If no deal is struck, ties will default to minimum standards set by the World Trade Organization, bringing higher tariffs and making onerous demands on business, which could weaken trade and dampen investment.