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.
"All ideas merit consideration, even the most far-fetched ones", Bruno Le Maire told Europe 1 radio, according the Reuters news agency. "Let's finish things that already underway before thinking of new ones".
The world's longest bridge is the Danyang-Kunshan Grand Bridge in China which opened in 2011 and spans 102.4 miles.
"The Strait of Dover is the world's busiest shipping lane, carrying more than 500 ships daily, so construction would cause huge disruption to sea traffic".
Last year Sunday Times Political Editor Tim Shipman revealed in his book Fall Out that Johnson had discussed building a road tunnel under the channel but was talked out of it by his aide Will Walden.More news: Montreal man fools police with fake vehicle made of snow
Plans for a Channel bridge were submitted in 1981, but dismissed as impractical due to the difficulties it would cause for vessels navigating busy shipping lanes, not to mention the difficulties of drivers switching to drive on the other side of the road.
Ian Ritchie, Royal Academician and architect, said: "Boris Bridge?"
Nigel Farage, the former leader of the nationalist U.K. Independence party, slammed the idea as unsafe and a waste of money.
Mr Johnson said in a tweet that the United Kingdom and France had agreed that a panel of experts should be assigned to look at "major projects". But what I would say is.we are going to have very good economic ties with France economically, culturally, in areas such as defence and security for many, many decades to come. "Should the Channel Tunnel be just a first step?"
Johnson has a history of proposing large-scale infrastructure projects. As mayor of London he backed a "garden bridge" over the River Thames, a project that was scrapped by his successor, Sadiq Khan, as poor value for money. Well, aside from Johnson's love of eye-catching infrastructure projects (remember the Boris Island airport?), this also rather fits into the theme of everyone trying to show that Britain will still be friends with European countries post-Brexit.