British Prime Minister Theresa May and European Commission chief Jean-Claude Juncker said they agreed after talks Monday to "accelerate" efforts for a Brexit deal ahead of a decisive EU summit this week.

May came to Brussels on short notice for a working dinner with Juncker as fears grow that negotiations on Britain's exit from the bloc in 2019 could end without an agreement.

In a short statement notably lacking on the key details at stake in the increasingly bad-tempered negotiations, the pair said their meeting took place in a "constructive and friendly atmosphere".

"The Prime Minister and the President of the European Commission reviewed the progress made in the... negotiations so far and agreed that these efforts should accelerate over the months to come," the statement said.

May and Juncker were joined at the dinner table by British Brexit minister David Davis and his EU counterpart, chief Brexit negotiator Michel Barnier, as well as the top aides to the two leaders.

Juncker had earlier declined to answer questions about the content of the meeting ahead of time, telling reporters in Brussels: "I will meet Mrs May this evening, we will talk and you will see the autopsy."

EU leaders meeting in Brussels on Thursday and Friday are due to decide whether or not negotiators can move on to discussing post-Brexit trade ties with Britain if "sufficient progress" has been made in divorce talks.

- Positive signal? -

The indication from the EU side is that they will postpone their decision to a summit in December as the break-up negotiations are currently deadlocked, particularly over the multi-billion-euro exit bill the EU says Britain must pay.

The EU has also demanded progress on the rights of three million European citizens in Britain, and on the future of Northern Ireland.

But Britain is still hoping for a positive signal that will ease growing concern, particularly on a two-year transition period to ease the "cliff edge" for businesses and citizens of a sudden departure.

EU President Donald Tusk, who will host the summit, warned last week that if there is no progress by December they will have to consider other options.

The EU leaders are set to agree at the summit that they will launch internal preparatory work now on the transition and future trade deal so they can move to the next phase quickly in December, a draft summit statement said.

"We are relatively close when it comes to the transition," a European source said on condition of anonymity.

But they toughened their conditions in the latest version of the draft under pressure from France and Germany, who want the bill to be settled before any movement on trade.

Ivan Rogers, who stepped down as Britain's ambassador to the EU in January, warned Monday that the government's aim to pull out of the single market and customs union would push the UK further out of the bloc "even than Turkey".

Britain's current stance means it will not be able to reach an accord similar to those the EU has with Norway and Switzerland, which are not members of the bloc but have access to the single market by meeting financial and free movement obligations, the ex-diplomat said.

Brussels' refusal to offer Britain unfettered access without agreeing to meet the same obligations "is not punishment, or vengeance, or spite... It's just an automatic consequence of our decision to exit," Rogers said in Glasgow.

- 'Father Christmas' -

With criticism of her Brexit strategy building up at home from both moderates and hardliners within her own Conservative party, May is hoping for a breakthrough that will bolster her position and ease some of the uncertainty around Britain's withdrawal.

"We've always said we want Britain leaving the EU to be a smooth process and this is part of achieving that," May's spokesman said of the 90-minute dinner.

The trip takes place as economic storm clouds gather for Britain, with companies warning they will have to start shifting some operations to the EU by the end of the year unless there is more certainty on Brexit.

May spoke to German Chancellor Angela Merkel on Sunday and the two agreed "on the importance of continued constructive progress in the UK's exit negotiations", according to a statement from the British prime minister's office.

She also spoke with French President Emmanuel Macron and Irish Prime Minister Leo Varadkar.

British Foreign Secretary Boris Johnson urged the EU to hurry up at a meeting with his counterparts in Luxembourg on Monday.

"Let's put a tiger in the tank, let's get these conversations going and stop letting the grass grow under our feet," Johnson said.

Luxembourg foreign minister Jean Asselborn told him to be patient.

"Father Christmas comes in December and there is also a summit then," he said.

source: AFP