London (AFP) - Prime Minister Theresa May will not "trap" Britain in an endless customs union with the European Union after Brexit, her office insisted Friday amid growing unease in her cabinet and party that this might be the price of a divorce deal.

Negotiations in Brussels have stepped up in recent days ahead of a high-stakes EU summit next week, with both sides seeking a breakthrough less than six months before Brexit in March 2019.

May briefed selected members of her cabinet late Thursday on the talks, at which several ministers reportedly expressed deep unease at a plan to avoid frontier checks with EU member Ireland.

Some eurosceptics, notably House of Commons leader Andrea Leadsom, are said to be considering quitting.

Britain has proposed that it continue to follow EU customs rules after Brexit as a fall-back option to keep open the land border with Ireland, until a wider trade deal is agreed that avoids the need for frontier checks.

May says this will only be temporary, but her spokeswoman was forced to clarify the point after media reports that the final "backstop" arrangement will have no legal end date.

"The prime minister would never agree to a deal which could trap the UK in a backstop permanently," she said.

The Downing Street spokeswoman repeated that Britain wanted a new trade deal by the end of December 2021 at the latest, although she declined to confirm the backstop would be "time-limited".

Her careful words only fuelled speculation of a compromise with Brussels, although Brexit Secretary Dominic Raab later said the backstop would have to be "finite", "short" and "time-limited".

Brussels has insisted that, as an insurance plan, the backstop cannot by its very nature have an end date.

However, eurosceptics in May's Conservative party are wary of being tied to the bloc indefinitely.

"That won't wash. The British people voted to take back control over money, laws, borders and trade," said former Brexit minister Steve Baker, one of a powerful group of eurosceptic Conservative MPs in parliament.

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- Threat to bring down government -

Amid speculation she could quit, Leadsom told the BBC: "We are at the final stages of a really complicated negotiation and I do think we have to give the prime minister the opportunity to be able to do a good deal."

But May is also facing a revolt by her Northern Irish allies over the long-term trading relationship, an outline of which London wants set out before Brexit.

The Democratic Unionist Party (DUP) has threatened to bring down the government if the deal results in new trade barriers between Northern Ireland and mainland Britain.

DUP MP Nigel Dodds said his party -- without which May has no majority in the House of Commons -- also needed any backstop to be time-limited.

"This is critical. If it isn't then the UK including Northen Ireland would be trapped in unacceptable arrangements unless and until the EU decide otherwise," he tweeted.

A full meeting of the cabinet is due next Tuesday, the day before May heads to Brussels, where she has been invited to address the other 27 EU leaders on her plans.

Both sides had set the October 18 summit as the deadline for a divorce deal, but the timetable is slipping, with another summit in November now looking likely.

European Commission head Jean-Claude Juncker told French newspaper Le Monde there must be "substantial progress".

"I want to believe that we will be able to find a deal with our British friends between the European Council meeting next week and the possible one in November," he said.

British finance minister Philip Hammond said there had been a "measurable change in pace" in the talks, but there were "some big differences left to resolve".

If a deal is done, he suggested Britain could enjoy a "deal dividend" which "should deliver us an upside in the form of higher economic growth and better outcomes".

London published its latest round of documents planning for a "no deal" Brexit scenario Friday, including warnings that Northern Ireland could face electricity supply disruptions and hinting at problems with Eurostar cross-Channel trains.