The chief negotiators for the UK and EU will continue post-Brexit trade talks in London until Wednesday.
Michel Barnier arrived in the UK on Thursday to restart negotiations with Lord David Frost after they stalled last week - but he was due to return home on Sunday.
EU sources told the BBC more talks are also planned in Brussels from Thursday.
Northern Ireland Secretary Brandon Lewis said the extended talks were "a very good sign" a deal can be done.
But he told the BBC's Andrew Marr: "We have got to make sure it is a deal that works, not just for our partners in Europe... but one that works for the United Kingdom."
The two sides are thought to be working on legal texts, but Whitehall sources have indicated major sticking points - like fishing rights and competition rules - remain unresolved.
The UK left the EU on 31 January but has been in a so-called transition period - continuing to follow EU rules and pay into the bloc - while the two sides hammer out a post-Brexit trade agreement.
The transition period is due to end on 31 December, but if a deal is not reached, the UK will trade with the EU on World Trade Organisation rules.
Some critics fear a no-deal scenario will cause problems for businesses, but the government insists the UK will prosper.
The EU had said a deal needed to be agreed by the end of October to allow time for it to be ratified by all the relevant parliaments, but UK Prime Minister Boris Johnson had warned of walking away from talks on 15 October.
After strong words from both sides and calls for "fundamental changes" to the approach to negotiations, a return to the table was agreed and Mr Barnier has been holding talks with UK chief negotiator Lord David Frost since Thursday.
On his arrival, Mr Barnier told reporters "every day counts" and the two sides shared a "huge common responsibility" in the talks.
The discussions had been expected to wrap up later on Sunday with the possibility of consequent conversations, but EU sources have told the BBC they will now continue in London for three more days, before moving to Brussels.
Mr Lewis said he was "always an optimist" around reaching a free trade agreement and he believed there was "a good chance we can get a deal".
But he told Andrew Marr: "The EU need to understand it is for them to move as well, so that we can get a deal that works for the UK as well - a proper free trade agreement that recognises us as the UK being a sovereign nation."
In line with a demand made by the UK, the talks resumed on all subjects based on proposed legal texts prepared by officials.
They also said that "nothing is agreed" until progress has been reached in all areas - which has been a key demand of the EU.
The two sides have been at odds over the issue of so-called "state aid" rules, which limit government help for industry in the name of ensuring fair economic competition.
The UK has rejected an EU demand made earlier in the year for it to continue following the bloc's rules on such subsidies as part of a trade agreement.
Lord Frost has suggested the UK could instead agree "principles" for how subsidies are spent - something welcomed by Mr Barnier on Wednesday.
The two sides are also haggling over how much European fishing boats should be able to catch in British waters from next year.
The EU has so far resisted UK demands for annual talks to decide stock limits, as well as a reduction in access for its vessels to British fishing grounds.