EU and UK officials have resumed Brexit talks in the hope of reaching a deal that can be agreed by leaders at a key summit on Thursday.
Prime Minister Boris Johnson is expected to update the cabinet on the progress of the negotiations, which continued into the early hours.
On Tuesday, there were unconfirmed reports that an agreement was imminent.
But government sources have now told the BBC that the chances of it happening this week are “shrinking”.
The EU’s chief negotiator, Michel Barnier, will update the bloc on the state of the negotiations later when he briefs EU commissioners and ambassadors.
Mr Johnson is facing a race against the clock to reach a new Brexit deal before the two-day gathering of EU leaders.
Any deal will need to be published – along with a legal text – if the EU27 are to consider ratifying the withdrawal agreement at their summit.
That meeting is crucial because under legislation passed last month – the Benn Act – Mr Johnson is compelled to ask the bloc for a delay to Brexit if he does not get a new deal approved by MPs by Saturday.
The UK is due to leave the EU at 23:00 GMT on 31 October and the prime minister has repeatedly insisted this will happen, regardless of whether there is a deal or not.
But Brexit Secretary Stephen Barclay told MPs on Wednesday that Mr Johnson “will comply with the law” regarding the terms of any further extension.
In addition to the challenges of reaching an agreement with the EU this week, Mr Johnson also requires support from Conservative Brexiteers and Democratic Unionists if he is likely to get his deal through Parliament.
Such support rests on the UK’s proposed alternative to the Irish backstop – the measure aimed at preventing a hard border on the island of Ireland.