Boris Johnson has said the British government should focus on stopping the Irish border becoming "significantly" harder after Brexit and suggested that crossings of the frontier could be monitored by technology like travel between London boroughs.
The British Foreign Secretary said "there's no border between Camden and Westminster" as he suggested that goods crossing between the Republic and Northern Ireland could be subject to electronic checks, in an apparent reference to the congestion charge.
Further details of his thinking on the issue are contained in a leaked letter to British Prime Minister Theresa May in which he suggests "it is wrong to see the task as maintaining 'no border'" but instead the aim was to stop the frontier becoming "significantly harder".
The letter, obtained by Sky News, suggested that "even if a hard border is reintroduced, we would expect to see 95% + of goods pass the border (without) checks".
The document from Mr Johnson, entitled "The Northern Ireland/Ireland border - the Facilitated Solution", accompanies a "concept note" that "draws on Foreign Office expertise".
The leak comes a day ahead of the publication of the European Commission's draft text for the withdrawal deal.
This will include procedures for putting into operation the "alignment" of Northern Irish regulations with the EU rulebook, which will be needed if no technological solution is found to keep the border open after Brexit.
Whitehall sources insisted that there was agreement the task was not about "no border" but "it's about no hard border".
Mr Johnson's comparison of the Irish border to north London was dismissed as "willful recklessness" and "unbelievable" by Labour MPs.
Mr Johnson also said that the CBI business lobby group and Labour leader Jeremy Corbyn were "wrong" to back a customs union with Brussels, as it would leave Britain a "colony" of the EU in a situation that would be the "worst of all worlds".
Mr Corbyn's initiative has set the scene for possible defeat for Mrs May at the hands of Tory rebels and Labour in an upcoming Commons vote on the Trade Bill.
But Mr Johnson told BBC Radio 4's Today programme: "You can't suck and blow at once, as they say, we're going to have to come out of the customs union in order to be able to do free trade deals."
And with the EU set to publish a legal document containing commitments to avoid a hard Irish frontier tomorrow, Mr Johnson dismissed the suggestion that leaving the tariff-free customs union would see the erection of border posts in Ireland.
"There's no border between Camden and Westminster, but when I was mayor of London we anaesthetically and invisibly took hundreds of millions of pounds from the accounts of people travelling between those two boroughs without any need for border checks whatever," he told BBC Radio 4's Today programme.
"It's a very relevant comparison because there's all sorts of scope for pre-booking, electronic checks, all sorts of things that you can do to obviate the need for a hard border to allow us to come out of the customs union, take back control of our trade policy and do trade deals," said Mr Johnson.
Responding, Labour anti-Brexit MP David Lammy tweeted "God help us all this isn't just stupidity and ignorance but wilful recklessness", while Paul Blomfield said it was "unbelievable".