In a policy paper, the government will outline proposals to allow Britain to maintain close economic and security ties with the European Union even after it leaves the bloc in March. "It is a vision that respects the result of the referendum (vote to leave), and delivers a principled and practical Brexit," Brexit Secretary Dominic Raab said in the foreword.

But the plan has caused outrage among eurosceptic members of May's Conservative party, and Raab's predecessor David Davis and foreign minister Boris Johnson dramatically quit this week in protest. Their departures, followed by a clutch of junior aides, destabilised May's government and revived talk of a leadership challenge against her.

The prime minister is also likely to face some opposition in Brussels, where officials have repeatedly warned Britain to lower its expectations about how close ties can be. May has briefed leaders including EU president Donald Tusk and German Chancellor Angela Merkel on her plan and reported a positive response, although they are awaiting the detail.

Britain does not have long to argue its case -- both sides are aiming for a deal by October, to allow time for its ratification by the British and European parliaments. Failure to agree would see Britain leave the EU without a deal, with the risk of huge economic disruption on both sides of the Channel.

Britain voted for Brexit in June 2016, but May has so far been unable to present a common position to Brussels on what she wants because of deep divisions in her government.

Amid warnings from businesses that continued uncertainty is risking investment and jobs, and fears time is running out, her cabinet finally agreed on a plan last week.

Britain would leave the EU's single market and customs union as planned, to end free movement of people and sign its own non-EU trade deals. But it would keep EU rules on goods to protect complex manufacturing supply chains, using technology to levy its own duties on UK-bound products from outside the bloc, while diverging on services.

"We need to rise to the challenge and grasp the opportunities" of Brexit, said Raab, adding that the policy represented a "balance".

But Johnson, a leading Brexit campaigner, said following EU rules without being able to alter them risked consigning Britain to the "status of colony", and said it looked like a "semi-Brexit".

Other eurosceptics who want a clean break with the bloc are also livid, prompting speculation they may launch a confidence vote against May. Brexit-backing MPs, including leading eurosceptic Conservative Jacob Rees-Mogg, will also seek to force her hand by tabling amendments to a trade bill being debated in the House of Commons next week. One would require a new law to join a new customs union, and another would demand the EU collect any tariffs set by Britain on UK-bound goods coming in from outside.

This last would likely be unacceptable to Brussels, thus killing May's plan, but Rees-Mogg told AFP his aim was only "to help the government stick to some of its earlier promises".