A post-Brexit trade deal is expected to be announced on Christmas Eve after talks between negotiators went on throughout the night.
It is understood that Britain and the EU have made progress on resolving issues including fishing rights, which have previously proved an obstacle on coming to an agreement, the PA news agency reported.
The late-night wrangling on the final aspects of the deal is thought to have included discussions on species of fish as part of the quota arrangements.
Irish foreign minister Simon Coveney suggested it was a “last-minute hitch” related to fishing that had delayed the agreement of a trade deal, which was expected early on Christmas Eve.
Speaking to Ireland’s RTE radio on Thursday morning, he said that Boris Johnson “had been due to hold a news conference around now”.
“That hasn’t happened. So there is some sort of last-minute hitch” related to “small text” of a fisheries agreement, Reuters reported him saying.
Just after midnight on Thursday, European Commission spokesperson Eric Mamer tweeted: “Brexit work will continue throughout the night.
“Grabbing some sleep is recommended to all Brexit-watchers at this point. It will hopefully be an early start tomorrow morning...”
The negotiations in Brussels were fuelled by a late-night delivery of pizzas.
Boris Johnson led a late-night call with his cabinet to update them on the situation.
Meanwhile, the PM is expected to have a call with European Commission president Ursula von der Leyen on Christmas Eve to agree the deal.
The expected deal comes with just days left before the current trading arrangements expire on December 31.
Any deal that Johnson secures is likely to pass through Parliament, with Labour not expected to oppose it. Keir Starmer has previously stressed that an agreement with the EU would be in the national interest.
On Thursday morning, Labour’s Hilary Benn – who is the chair of the Commons Brexit committee – said he had “no doubt” Parliament would approve legislation for a deal if one was brokered before January 31.
He told BBC Breakfast: “The alternative is no-deal and that really doesn’t bear contemplation at all because of the damage it would do to the economy.
“What any deal is going to do is to make the consequences of Brexit for business less bad than they would otherwise be.
“Remember this is the first trade deal in history where one party has gone in knowing it will come out with worse arrangements than it went in with.”
However, the prime minister is set to face scrutiny from his own benches, with the European Research Group (ERG) of hardline pro-Brexit Tory MPs having said they would go through any deal with a fine-toothed comb.
The ERG said it would reconvene its so-called “star chamber” of legal experts to examine the text.
A statement issued by the group on Wednesday said: “Given that the new agreement is also highly complex, the star chamber will scrutinise it in detail, to ensure that its provisions genuinely protect the sovereignty of the United Kingdom after we exit the transition period at the end of this year.”
If a deal is agreed it would also have to be backed by the EU’s 27 member states.
MPs and peers would be recalled to Parliament to vote on a deal next week, but the European Parliament has already said it will not have time to ratify a deal before January 1 – meaning any agreement is likely to be provisional.