Boris Johnson has told Brexiteers they should not fear betrayal as the Government draws up a "backstop" scheme which would keep the UK in the EU's customs union beyond 2020.
He called on the Leave camp to give Theresa May "time and space" to negotiate a deal that delivers on her promise to take Britain out of the customs union and single market and enable it to strike new trade deals around the world.
But his comments will be seen as a thinly veiled warning to the Prime Minister that she must not let the so-called "backstop" become a permanent solution.
There was dismay among committed supporters of EU withdrawal at reports that Mrs May's Brexit war cabinet has discussed a plan under which the whole of Britain would observe EU external tariffs until the practical arrangements are in place to keep the Irish border open.
There are fears in the Leave camp that UK involvement in EU structures beyond the conclusion of the transition period in December 2020 may end up being indefinitely extended.
But Mr Johnson insisted that he is convinced the Prime Minister will be "true to her word" and deliver a final settlement in which the UK is not tied to the EU's system of common external tariffs.
The Foreign Secretary is understood to be among two ministers who argued against the backstop option in last week's meeting of the Cabinet sub-committee overseeing Brexit strategy and negotiations.
The scheme was devised as an alternative to the European Commission proposal that Northern Ireland should remain in the customs union if no better resolution for the border issue can be found.
This Brussels backstop was roundly rejected by Mrs May as something no British PM could sign up to, as it effectively draws a customs border down the Irish Sea.
While he regards the new proposal as preferable to the Commission's scheme, Mr Johnson left no doubt he believes neither will be necessary, as Mrs May will be able to negotiate a satisfactory deal before the time comes when they would have to be implemented.
The Foreign Secretary's own preference is for the "maximum facilitation" scheme - known as Max Fac - which would use trusted trader arrangements and technology like number plate recognition cameras to avoid the need for border checks.
Speaking to reporters during a tour of Latin America, the Foreign Secretary said: "Brexiteers fearing betrayal over the customs backstop must understand that the PM has been very clear that neither option is an outcome we desire - we want a deal with the EU and she will deliver it.
"I'm convinced that the Prime Minister will be true to her promises of a Brexit deal - that sees Britain come out of the customs union and single market, have borders as frictionless as possible, reject European Court of Justice interference, control immigration and free to conduct unhindered free trade deals across the world.
"We must now give the Prime Minister time and space to negotiate this Brexit vision."
Mr Johnson's five-day visit, taking in Peru, Argentina and Chile, is designed to pave the way for post-Brexit deals with three of the region's countries most committed to free trade.
Peru and Chile already have free trade agreements (FTAs) with the EU, which the UK hopes to roll over and enhance after Brexit.
But Argentina is part of the Mercosur grouping, alongside countries like Brazil, which has failed in many years' efforts to strike an FTA with the EU in the face of European farmers' resistance to the influx of vast supplies of cheap agricultural produce like beef.
Mr Johnson insisted Brexit will provide opportunities to increase Britain's share of import markets, which currently stands at around 1% in all of the countries he is visiting.
"Already during my time in South America I've been bowled over by the optimism and excitement from nations keen to forge deeper ties and new trading relationships with the UK," he said.
"Make no mistake, we will be ready to take advantage of these opportunities when we leave the EU."