Ian Frith, the former president of the Institution of Structural Engineers, told BBC Radio 4's Today programme that a bridge from the United Kingdom to France would be "a huge undertaking, but absolutely possible".
The Foreign Secretary said "good connections" were important between the two countries, and suggested the Channel Tunnel might simply be "a first step".
Johnson made the radical suggestion during Thursday's Franco-British summit near London, a source in the French presidency said, prompting reactions ranging from polite diplomatic putdowns to bemusement to scorn.
Experts were quick to pour scorn on the idea.
The Foreign Secretary - famed for championing grandiose building schemes - raised the prospect of a 22-mile crossing after a UK-France summit attended by Theresa May, President Emmanuel Macron and senior ministers from both countries.
The Daily Telegraph reported him telling the French leader that it was "ridiculous" that the neighbours were "linked by a single railway".
Massachusetts Democrats accuse GOP of failed leadership as government shutdown looms
Pennsylvania's senior senator is taking aim at the Republican Party for the potential shutdown of the federal government. The House voted Thursday night to keep the government open through mid-February, and now the matter moves to the Senate.
"The Channel Tunnel and the ferry routes are working well within capacity, so it makes no sense to commit huge amounts of taxpayers' money in an uncertain economic climate to a costly bridge project that we don't need".
To be clear: This appears to just be an idea at this stage, rather than a formal proposal.
For starters, its feasibility would be highly dependent on its precise location.
It's not actually clear what bridge Johnson is referring to here, and according to a CNN article published in 2017, the Guinness World Records recognises the Lake Pontchartrain Causeway in Louisiana, USA as the longest bridge over continuous water.
And in fact the idea of a bridge across the Channel is nothing new.
It was unclear how such a structure might work in one of the world's busiest shipping lanes, or whether its construction might interrupt trade.
Conservative MP Mark Pritchard took to Twitter to say that Johnson's idea was "a bridge too far".