Boris Johnson has sent a clear message to Theresa May that any deal for leaving the EU must involve "the whole of the UK taking back control" as negotiations over the Irish border continue.
In an apparent warning against offering special status to Northern Ireland or other areas of the country, the Foreign Secretary said that all parts of Britain must have control over the country's borders, laws and money.
He said it was time to "get going" with trade talks, which Mrs May is hoping to achieve when she attends a meeting of the European Council next week.
Negotiations between London, Dublin, Belfast and Brussels continued today as Mrs May tried to rewrite a draft agreement on the terms of Britain's withdrawal that all sides can sign up to.
Mrs May was forced to abandon talks with the European Commission president Jean-Claude Juncker on Monday after the DUP, on whose votes Mrs May relies for her majority, reacted with fury to a passage in a draft agreement that called for "regulatory alignment" between Northern Ireland and the Republic of Ireland.
DUP and Tory MPs said the wording meant that either Northern Ireland was going to leave the EU on different terms to the rest of the UK or the whole of the UK was heading for a "soft" Brexit by yoking itself to Brussels.
Answering questions after a foreign policy speech in London, Mr Johnson said: "Whatever way we devise for getting onto the body of the (Brexit) talks, it's got to be consistent with the whole of the United Kingdom taking back control of our laws, of our borders and of our cash," Mr Johnson told reporters after a speech.
"What we want to achieve is a new relationship, a deep and special partnership in which we can intensify our trade links and continue to work together on foreign policy and security."
"But to achieve that we need to get going with the second part of the talks. That's the exciting bit," he said.
Monday's abandoned attempt to reach a deal was followed by calls from Nicola Sturgeon, the first minister of Scotland, and Sadiq Khan, the Mayor of London, for similar separate arrangements for other parts of the United Kingdom that voted to remain in the EU.
Mrs May was accused of trying to “bounce” the Cabinet into agreeing to “regulatory alignment” between Ulster and Ireland after it emerged she did not brief senior ministers before talks in Brussels on Monday that stalled over the controversial issue.
Mr Johnson on Wednesday defended Mrs May's "very good" offer to settle the so-called Brexit bill, which is expected to amount to up to £50 billion, and said Britain should be "punctilious" about paying reasonable obligations.
"I was asked my reaction to some of the very extortionate sums that I had heard in the region of £80 or £100 billion, and, I don't want to repeat the offending phrase, but go whistle seems the appropriate reaction to that kind of sum."
"When it comes to other sums and other obligations, a more detailed examination of our obligations, our financial obligations, I think you will find the British Government is absolutely punctilious in wanting to meet our friends more than half way.
"I think that you will find the financial offer that we are making is very good," he said.
Mr Johnson made the comments following a speech laying out his vision for British foreign policy in the Middle East after Brexit.
He said Britain must dare to step back into some of the world’s most intractable conflict zones in the Middle East to avoid decades of further terror and instability, and should seek in boost its military, political, and diplomatic engagement there.
He also condemned Donald Trump's decision to recognize Jerusalem as the capital of Israel and move its embassy to Jerusalem before a peace deal is reached with the Palestinians.
"The UK believes this is unhelpful in terms of prospects for peace in the region," he said. "The British Embassy to Israel remains in Tel Aviv and we have no plans to move it."
"We ... think that the future of Jerusalem must be settled as part of the negotiated agreement between Israel and the Palestinians and as part of the two-state solution," he said.
"This decision, having been announced by President Trump, the world would like to see some serious announcements by the U.S. about how they see the Middle East peace process and how to bring the two sides together."
He also confirmed that he would be travelling to Iran to seek to resolve "very difficult consular cases" including that of Nazanin Zaghari-Ratcliffe, the British-Iranian woman jailed there on suspicion of spying - charges she denies.