Johnson called the bridge proposal, which could see a 20-mile long crossing built between Portpatrick in Scotland and Larne in Northern Ireland, “a very interesting idea”.
But Labour have called the notion “a distraction”, and many of Mr Johnson’s critics have begun resharing a blistering attack by James Duncan, a retired offshore engineer from Edinburgh.
In a letter to the Sunday Times in 2019, Duncan labelled the bridge “another thoughtless soundbite”, and said “no… responsible government would consider building such a bridge”.
“As a retired offshore engineer, I know this is about as feasible as building a bridge to the moon.
“Many long bridges have been built,” he wrote, “but none across such a wide, deep and stormy stretch of water. For a great part of the 22-mile route the water is more than 1,000ft deep.”
Duncan claimed that the bridge “would require about 30 support towers at least 1,400ft high to carry the road deck across the deepest part and above the shipping channel. In total the bridge would require 54 towers, of heights never achieved anywhere in the world.”
The former engineer paid particular attention to the challenge of Beaufort Dyke, a 30 mile long and two mile wide trench in the Irish Sea, used by the British military to dispose of old explosives after the end of the Second World War.
“The Ministry of Defence estimates the total dumped at more than 1.5m tons. There are no maps of their locations,” he wrote.
“No sane contractor or responsible government would consider building such a bridge,” said Duncan, “and because of the weather conditions it would probably have to be closed for considerable periods if it did.”
“The proposal is just another thoughtless soundbite,” he added, “this is typical Johnson.”
Despite the criticism, a spokesperson for Number 10 said: “The prime minister has said it would have some merit - as a result you would expect government to be looking into it.
“Work is underway by a range of government officials.”
Details of the plan for the bridge are still unclear, but it is rumoured to cost around £20bn.
Experts should deliver their report of the project’s feasibility later this year, according to reporting by the BBC’s Nick Eardley.