Boris Johnson has dismissed concerns that leaving the EU customs union after Brexit could lead to a hard Irish border by suggesting that crossings of the frontier could be monitored by technology like travel between London boroughs.
The UK's Foreign Secretary said "there's no border between Camden and Westminster" as he suggested that goods crossing between the Republic of Ireland and Northern Ireland could be subject to electronic checks, in an apparent reference to the congestion charge.
But his suggestion was dismissed as "willful recklessness" and "unbelievable" by Labour MPs in England.
Mr Johnson also said that the Britain's 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."
"There's no border between Camden and Westminster."— BBC Radio 4 Today (@BBCr4today) February 27, 2018
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 said.
"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".
Did our Foreign Secretary Boris Johnson really just compare the border between Northern Ireland and the Republic of Ireland to the border between Camden and Islington on @BBCr4today just now? God help us all this isn’t just stupidity and ignorance but wilful recklessness #r4today— David Lammy (@DavidLammy) February 27, 2018
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."
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. https://t.co/Mok7Byylju— David Lammy (@DavidLammy) February 27, 2018
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!"
And Walthamstow Labour MP Stella Creasy wrote: "A foreign secretary who literally has no conception of what foreign actually means - that is what brexit has brought us to as a nation. Please, please JUST MAKE IT STOP...."
Mr Johnson spoke as Britain's International Trade Secretary Liam Fox prepared to give a speech in which he will describe Mr Corbyn's plan to stay in the customs union as "a complete sell-out of Britain's national interests" which will leave the UK in a worse position than it is now.
In the latest of the Cabinet's "road to Brexit" speeches in London, Dr Fox will warn Tory rebels that the arrangement would constrain the UK from taking advantage of future trade opportunities and leave the country as "rule-takers" from Brussels.
But his argument was dismissed by the former top civil servant in his department, who 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 Donnelly, 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.
He told Today: "You're giving up a three-course meal, which is the depth and intensity of our trade relationships across the European Union and partners now, for the promise of a packet of crisps in the future if we manage to do trade deals outside the European Union which aren't going to compensate for what we're giving up.
"You just have to look at the arithmetic - it doesn't add up I'm afraid."
Mr Johnson said he disagreed "very strongly" with Sir Martin "of the Brussels commission", arguing that there is an "insatiable" market for UK services outside the EU.
The Foreign Secretary also rejected reports that the EU is set to demand the European Court of Justice is the ultimate arbiter in treaty-related disputes as it would not amount to "taking back control".
And he suggested he was growing weary of talking about Brexit, despite leading the Vote Leave campaign in 2016's referendum.
"I think there's a very good deal to be done, one day we'll be sitting here not talking about Brexit, it's going to be fantastic, but one day we will be able to do a deal I think that reflects what the Prime Minister set out at Lancaster House and Florence and elsewhere, around which the whole Cabinet united last week."