She added: "We expect that the end-state customs arrangement at the latest will be in by December 2021 and we will be working to make sure that it is in earlier than that".
Prime Minister Theresa May, who is struggling to contain divisions within her cabinet over Brexit, proposed on Thursday that she would keep Britain tied to the EU's customs union for up to extra year after an nearly two-year transition period if there were any delay in implementing a divorce deal. Is it an all-weather backstop?
Rumours were flying around all morning that the Brexit secretary David Davis might resign unless the customs arrangement had a clear time limit.
The prime minister also held separate face-to-face discussions in her parliamentary office with the two other leading Brexiteers, foreign secretary Boris Johnson and global trade secretary Liam Fox. None of the three Cabinet ministers threatened to resign during their meetings with the PM, Mrs May's spokeswoman said.
The Prime Minister, who has been beset by problems over the Brexit process in recent days, said the United Kingdom could look forward to life after March 2019.
A source close to Davis told HuffPost UK: "Obviously there's been a back and forth on this paper, as there always is whenever the government publishes anything".
The assurance came after May held a series of meetings with Brexit supporters in her Cabinet amid reports that David Davis, the minister leading negotiations with the European Union, had threatened to resign because an earlier proposal could have forced Britain to follow the bloc's trade rules indefinitely.
The EU's chief Brexit negotiator was giving his first substantial reaction to the U.K.'s alternative "backstop" proposal to avoid a hard border in Northern Ireland if negotiators fail to reach agreement. "How does that fit with the absence of a hard border in all circumstances?" Does it respect the integrity of the SM/CU (single market / customs union)?
The UK will propose the customs arrangement - widely known as a "backstop" plan - to ensure people and goods can still move between Northern Ireland and the Republic of Ireland after Brexit occurs in March next year.
"The backstop paper has been amended and now expresses, in much more detail, the time-limited nature of our proposal - something the prime minister and David Davis have always been committed to".