Work is under way to develop Boris Johnson's plans for a new bridge to be built linking Scotland and Northern Ireland, Downing Street has announced.
The prime minister’s spokesman confirmed "a range" of officials are fleshing out the proposal.
It was first touted by Mr Johnson when he was foreign secretary.
"What we need to do is build a bridge between our islands. Why don't we? Why don't we?” he told the Sunday Times in 2018
"There is so much more we can do, and what grieves me about the current approach to Brexit is that we are just in danger of not believing in ourselves, not believing in Britain."
Mr Johnson later put a price of "about £15bn" on building the physical link stretching from Stranraer in Scotland to Larne in Northern Ireland.
Now Number 10 has revealed the idea is being developed by the civil service.
"Work is underway looking into it - it’s being done by a range of government officials,” Mr Johnson’s spokesman confirmed to journalists on Monday.
"The prime minister is ambitious in terms of infrastructure projects around the UK to boost connectivity.
"There is a proper piece of work being carried out into the idea.
"It is being done by a range of government officials; as with all work we ask for and it reports in to Number 10."
The idea has been criticised by Theresa May’s former infrastructure tsar and Labour peer Andrew Adonis.
“So now Boris wants to build a garden bridge all the way to Belfast,” he tweeted.
“If you think HS2 has problems, multiply by 10 and divide the benefits by 100.”
Mr Johnson also faced criticism for his role overseeing a failed garden bridge and other projects in London while he was mayor in the capital city.
Labour MP Wes Streeting said: “We’ve been here before with vast sums of public money wasted on vanity projects like the cable car, garden bridge, and ‘Boris island airport’. The man never learns.”