Boris Johnson has said the 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 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 the Prime Minister 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 the Foreign Secretary, 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 with the Republic 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 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 Theresa 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 on Wednesday, Mr Johnson dismissed the suggestion that leaving the tariff-free customs union would see the erection of border posts on the island.
"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 Today.
"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."
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".
Mr Johnson's border comments were mocked by Tottenham Labour MP Mr Lammy, who said on Twitter: "When I was a young boy we were told to stay away from the Troubles on the Caledonian Rd & marching bands in Regent's Park. The Chalk Farm Peace Agreement has brought peace in our time. People can get the tube from Camden Town to Finsbury Park without being searched at the border."
Sheffield Labour MP Mr Blomfield said: "Stumbling, bumbling borisjohnson compares north & south of Ireland with Islington & Camden on r4Today while trying to explain his frictionless border without a Customs Union. Unbelievable!"
Meanwhile International Trade Secretary Liam Fox became embroiled in a row with a former top official at his department over the Government's Brexit plans.
Sir Martin Donnelly said leaving the customs union to strike free trade deals around the world was like "giving up a three-course meal for the promise of a packet of crisps".
Sir Martin, who left his role as permanent secretary at the Department of International Trade last year, said any divergence from Brussels' rules would deal a blow to British services which would not be compensated for through deals with nations such as the US.
But Dr Fox, answering questions after a speech in London, said: "It is unsurprising that those who spent a lifetime working within the European Union would see moving away from the European Union as being threatening."
The International Trade Secretary said the UK could reach agreements with the EU as well as other nations.
"It is not a choice of one or the other. And, in any case, I think the UK Brexit process is, as we have all discovered, a little more complex than a packet of Walkers."
