London  - Britain is moving closer to an agreement on withdrawing from the European Union, based on Prime Minister Theresa May's "Chequers" proposals, Brexit Secretary Dominic Raab said on Tuesday.

Raab told parliament his government was "closing in on workable solutions to all of the key outstanding issues."

Eurosceptic lawmakers oppose May's plan

He said negotiators were "building on the progress we made during the summer on issues such as data and information, the treatment of ongoing police and judicial co-operation in criminal matters and ongoing union judicial administrative procedures after the date of exit."

"We have also been discussing our proposals on the linkage needed between the withdrawal agreement and the future relationship, and the EU is engaging constructively," Raab said.

Former Brexit secretary David Davis, who resigned in July to oppose May's plan, earlier Tuesday published an open letter telling the 315 Conservative lawmakers that the Chequers proposals offered "none of the benefits of Brexit."

Davis urged the lawmakers to push for a looser, Canada-style free trade deal with the EU.

"If we stay on our current trajectory, we will go into the next election [in 2022] with the government having delivered none of the benefits of Brexit, with the country reduced to being a rule-taker from Brussels," he wrote.

Another leading Conservative rebel warned on Tuesday that dozens of eurosceptic lawmakers opposed to May's plan are likely vote against it if, as expected, it is presented to parliament next month.

Steve Baker told BBC Radio 4's Today programme that at least 40 pro-Brexit Conservatives will not accept the kind of "half in, half out" deal for leaving the European Union that May appears to be close to agreeing with EU leaders.

"Other options should be explored"

Baker was responding to reports that May could reach a deal to keep an open Irish border after Britain leaves the EU in March by keeping the current customs arrangements beyond a 21-month transitional period.

In Brussels, meanwhile, Northern Ireland's Democratic Unionist Party (DUP) warned that it could also oppose May in parliament if she accepts the EU's "untenable" backstop plan for the Irish border, which could involve checks on some goods transported to Northern Ireland from Britain.

"There cannot be barriers to trade in the UK internal market which would damage the economic well-being of Northern Ireland," DUP leader Arlene Foster told reporters following talks with Michel Barnier, the EU's chief Brexit negotiator.

"And therefore we could not support any arrangement which would give rise to either customs or regulatory barriers within the UK internal market," said Foster, whose 10 DUP members in the British parliament are key to May's minority government.

Foster also suggested that other options should be explored, rather than just seeing a choice between the current proposals and a "no deal" Brexit.

May said in June that her cabinet had agreed a compromise proposal for a temporary, last-resort "backstop" arrangement to maintain free movement of goods and people across the border after the transitional period.

She has insisted any backstop would only be a last resort that should end by December 2021.