The Prime Minister has been accused of wasting public money as his rivals seek to sink his plan for a bridge linking Scotland and Northern Ireland.
The merits of a potential road link across the Irish Sea, at a reported cost of £20bn, are being explored by Government officials.
But Brexit Party leader Nigel Farage slammed the proposal as "absolutely mad".
Speaking on his LBC radio show, Mr Farage predicted that such a project would exceed both its budget and deadline.
"I think the whole thing is absolutely mad," he said.
"We need more roads, not smart motorways that are going to kill people.
"We need to be able to get from east to west across the country.
"I think this is a huge mistake. These projects always become massively over time and massively over budget."
Conservative Peer Lord Michael Forsyth has added his own criticisms.
A former Scottish Secretary, Lord Michael branded the notion "ridiculous" and said the money would be better put to use on improving existing roads.
"I left office in 1997 and we were talking about the dualling of the A1 from Edinburgh and Newcastle then, and it's still not dualled," he told the BBC.
"The A96 is another long-standing saga where people lose their lives.
"So if there's the money available to spend billions to build a bridge ... then let's actually spend it on the things that actually matter to local economies and local people and to have infrastructure that is actually related to our economic needs. Frankly, I don't think that applies to a bridge across the Irish Sea."
But architect Alan Dunlop said the media in China has been left bemused by the reports because the country "looks more positively" on such projects.
His plans were inspired by the world's longest sea bridge linking Hong Kong to Zhuhai-Macau.
"China is different because it can see a pathway into how to do this - it has resources and bridges built by British engineers," he said. "It has no issue in bringing in expertise from other countries."
Mr Dunlop said the UK could learn from "the more ambitious attitude" of the Chinese.