Brexit Secretary Dominic Raab is meeting the EU’s chief negotiator in an attempt to break the deadlock in talks.
He and Michel Barnier are set for six hours of talks in Brussels ahead of the crucial EU summit on 17 October.
Both sides are hoping to agree a divorce deal and a statement on future trading relations before the summit.
But ex-Foreign Secretary Boris Johnson said Theresa May’s plan to follow EU rules on trade in goods, unpopular with many Tory MPs, must be “chucked”.
Mr Johnson, who quit the cabinet in July in protest over the PM’s proposals, hammered out at her country residence Chequers, said he agreed with Mrs May’s former aide Nick Timothy, who urged her to ditch the plan.
Mr Timothy wrote on Thursday that she would have to make further concessions to get EU backing for it and this would be unacceptable.
And Stuart Jackson, the former Tory MP who until recently was an aide to Dominic Raab’s predecessor David Davis, said the Chequers plan “has not really got any friends” on either side.
“For the EU, it’s outside their core principles and undermines the integrity of the single market,” he told BBC Radio 4’s Today. “In the UK, people believe it is Brexit in name only.”
The UK is due to leave the EU on 29 March 2019.
Mr Raab vowed to increase the frequency of talks with Mr Barnier when he took over the job in July, following the resignation of David Davis.
The meeting comes after the most senior member of Theresa May’s cabinet, David Lidington, called for the EU to back the prime minister’s Brexit plan or risk a no deal scenario.
On Thursday evening, Mr Raab met with the European Parliament’s Brexit representative, Guy Verhofstadt.
The former Belgian prime minister called the meeting “productive”, tweeting that he wanted a “close future association” with the UK after Brexit that “respects the EU’s core principles”.
Mr Raab told a Lords Committee earlier this week that he had a “good professional and personal rapport” with Mr Barnier, adding that he was “confident that a deal is within our sights”.
But he suggested that the October deadline could slip, saying there was a “possibility that it may creep beyond” that date.
Speaking in Berlin on Wednesday, Mr Barnier said the EU was “prepared to offer Britain a partnership such as there never has been with any other third country”.
But, he added, it would not permit anything that weakened the single market.
“We respect Britain’s red lines scrupulously. In return, they must respect what we are. Single market means single market… There is no single market a la carte,” he told reporters.
France’s European minister told the BBC the consequences of the UK leaving without a deal would be “tough” for both sides but neither did the EU want a “bad deal” just for the sake of it.
Nathalie Loiseau said the UK could not expect a deal which gave it both the economic “benefits” of a Norway-style association agreement with the EU but the lesser “obligations” of a Canada-style free trade deal.
“There is something in-between but there has to be a balance between the rights and obligations in relations with the EU,” she told BBC Radio 4’s Today.