Theresa May is set to publish a Brexit blueprint to outline her priorities for Britain’s future relationship with the EU before a critical summit in Brussels next month.
The white paper was described as the “most significant publication on the EU since the referendum” by Brexit Secretary David Davis as he announced the plans on Tuesday.
The paper will include “detailed, ambitious and precise explanations" of the Government's positions, Mr Davis added.
It comes after Prime Minister Mrs May’s Brexit "war cabinet" met again on Tuesday without reaching an agreement on two possible options for customs arrangements.
The EU is piling pressure on Britain to present its preferred option at the upcoming meeting of the European Council in June.
Downing Street insists it will not put a timetable on the process.
Mr Davis said the document would set out "what would change and what would feel different" after Brexit.
He said it was an opportunity for the Government to show the thought behind its approach to the change to a domestic and EU audience.
He said: "This will be our most significant publication on the EU since the referendum.
"It will communicate our ambition for the UK's future relationship with the EU, in the context of our vision for the UK's future role in the world."
Mr Davis reportedly told Mrs May that the customs partnership model she favours would be illegal under international law.
The Times said he wrote to Mrs May setting out his fears and had the backing of other Brexit-supporting Cabinet ministers.
It said the attorney general's office was now looking into the legality of both proposals ahead of a final decision by the Cabinet.
Theresa May heads to Bulgaria following PMQs on Wednesday to attend the EU-Western Balkans summit in Sofia, where she will attend a working dinner with the other 27 EU leaders.
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She will then fly to Macedonia for bilateral talks with prime minister Zoran Zaev.
Conservative MPs were invited into Downing Street on Monday for a briefing from the PM and chief of staff Gavin Barwell on the "customs partnership", believed to be Mrs May's preferred option, under which the UK would collect tariffs on the EU's behalf, and the "max fac" scheme, which involves the use of technology to minimise friction at the Irish border.
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But EU chief negotiator Michel Barnier is reported to have told a Brussels meeting that it was not worth fighting about the two UK proposals, as neither of them is "realistic".
Labour said on Tuesday it was launching a new parliamentary bid to force the Government to release details of its proposals for post-Brexit customs arrangements on the Irish border.
Jeremy Corbyn's party has tabled a motion in the Commons for debate on Wednesday designed to break the "deadlock".
It would require the Government to release to Parliament all papers prepared for the sub-committee on the two customs models, including any economic analysis.
It is the latest in a string of Labour motions using an arcane parliamentary procedure to make the vote binding on the Government by issuing a "humble address" to the Queen asking her to require ministers to comply.
Shadow Brexit minister Paul Blomfield said it was "deeply disturbing that, after yet another meeting, the Cabinet still cannot agree on the most fundamental Brexit issues".
He added: "Ministers have finally agreed to publish a white paper on the Government's negotiating position, but they still don't know what it will say.
"Labour called for a white paper before Article 50 was triggered.
"However, ministers have wasted months arguing amongst themselves rather than negotiating in the national interest.
"Today's failure highlights the deep division at the heart of Government on the most basic of issues.
"Whether those divisions can be resolved in the next month remains to be seen. If the Cabinet can't take the decisions, Parliament will."
