Irish politicians have rounded on Boris Johnson for comparing the border with Northern Ireland to divisions between London boroughs.
The Foreign Secretary's suggestion that technology was the solution to the frontier question were mocked in political circles in Britain and drew disdain in Dublin and Belfast.
Stephen Donnelly, the Brexit spokesman for the opposition Fianna Fail party, recalled his years working in London after hearing Mr Johnson's remarks.
"I lived in Camden for several years, and was never stopped crossing the 'border' to Islington," he said.
"I have, however, had military rifles pointed at me when crossing into Northern Ireland in the 90s. Suggesting these borders are the same is extraordinary."
Mr Johnson suggested crossing the Irish border could be solved in the same way technology was deployed to collect congestion charges between Camden and Westminster when he was London mayor.
SDLP leader Colum Eastwood said he wanted to educate the Foreign Secretary about the border issues.
"When Boris Johnson decides to come down from the other planet that he clearly inhabits, he should visit the Irish border and see the scale of the challenge we're facing with his own eyes before making further pronouncements," the nationalist leader said.
The SDLP said it sent a memo to the Foreign Office detailing the difficulties with a hard border and the realities faced by people, businesses and communities.
"Trivialising the very serious concerns relating to Ireland displays a dangerous ignorance that must be challenged," Mr Eastwood said.
Mr Johnson's much-criticised analogy came hours after Prime Minister Theresa May and Irish Taoiseach Leo Varadkar spoke on the phone in what will be a hugely important week in Brexit talks.
The two leaders held discussions as EU chief negotiator Michel Barnier prepares to publish a legal document expected to cement the December agreement that should ensure no hard border on the island of Ireland when the UK leaves Europe.
Under London's post-Brexit trade plan the UK would examine existing regulations and decide whether it wants to maintain, alter or abandon them in certain areas.
Sinn Fein's Brexit spokesman David Cullinane branded the Foreign Secretary's intervention as "silly" and "ignorant".
"It is not surprising that Boris Johnson would make those kind of silly comparisons because he and the hard Brexiteers simply do not want to face up to the reality that any type of Brexit or any type of exit from the customs union and the single market for Britain and the north will mean a hardening of the border," he said.
"I do not think it will be a surprise for many people that Boris Johnson would make an ignorant observation when it comes to Ireland."