Boris Johnson's proposals for a new bridge spanning the English Channel could cost up to £120 billion, according to experts.
Mr Johnson previously hinted at the prospect of Britain and France building a bridge over the English Channel.
The Foreign Secretary said "good connections" were important and that it was ridiculous that two of the world's biggest economies were only linked by a single railway.
Alan Dunlop, who teaches architecture at the University of Liverpool, told The Times: "I'd say at least £120 billion for a Channel bridge and that's a conservative estimate.
"It would really be cheaper to mover France closer."
Meanwhile Dr Kostas Tsavdaridis, associate professor in Structural Engineering at the University of Leeds, said constructing such a bridge was "doable" so long as issues including waves, water flow and depth, wind strength and soil condition were properly planned for first.
However issues including the weather, shipping and the financial implications could make the concept a less-than-practical reality.
It is thought that a bridge could be challenging for vessels crossing one of the busiest shipping channels in the world, sometimes with limited visibility.
Dr Tsavdaridis said he would use such a bridge "but only on the good days of the year", as conditions could be harsh.
He said any seismic action in the area would not have a significant effect on the structure, and it was being mooted for the narrowest and shallowest stretch of the channel.
At 20-plus miles, it would be one of the longest bridges, but "it has been done in the past", he said.
Trade body the UK Chamber of Shipping said in a tweet: "Building a huge concrete structure in the middle of the world's busiest shipping lane might come with some challenges."
The world's longest bridge is the Danyang-Kunshan Grand Bridge in China, which spans just over 100 miles.
Additional reporting by the Press Association
