Reports at the weekend suggested Boris Johnson remained keen on an ambitious plan for the bridge, which could cost between £15-20 billion to complete and would be one of the longest in the world.
The plan was condemned by the SNP as an “unsubstantiated vanity project”, and the government was unable to say whether it had consulted Scottish ministers, who would be responsible for planning permissions, traffic management and any access roads.
It is also unclear how goods transiting over the bridge would be regulated after Brexit, with companies trading across the Irish Sea braced for new paperwork once EU single market regulations no longer apply.
Downing Street said a "range of officials" were looking at the idea of a bridge, but were unable to say how many civil servants were examining proposals.
The Prime Minister's official spokesman said it was a "proper piece of work" and Mr Johnson was "ambitious" about infrastructure projects across the country.
Mr Johnson has repeatedly spoken about the prospect of a bridge, even though experts have warned that the depth of the Irish Sea and the presence of dumped munitions would cause problems for any project.
The Prime Minister's spokesman said: "The PM set out this was an idea which he believed could have some merit so, as a result of that, you would expect the Government to be looking into it."
The scoping work is being run from Number 10, with a range of officials reporting into it.
"The PM is ambitious in terms of infrastructure projects," Mr Johnson's spokesman said.
"He's looking at a wide range of schemes across the UK which could improve connectivity."
The Prime Minister told MPs "watch this space" when asked about the prospect of a "Boris bridge" in Parliament in December 2019.
And in November 2018, he said: "The problem is not the undersea Beaufort's Dyke or lack of funds. The problem is an absence of political will."
The distance from Larne to Portpatrick, one of the most likely routes for a bridge, is around 45km.
Government sources said it was possible to build a bridge which has a tunnelled section to cope with some of the difficulties caused by the depth of the Irish Sea - a similar design to the Oresund Bridge linking Denmark and Sweden, made famous worldwide by the television series ‘The Bridge’.
SNP spokesperson: “It is going to take more than a bridge to undo the harm to EU relations that the Tory government has caused with its extreme Brexit plans, and given Boris Johnson’s failed history of unwanted and over budget bridge projects we are going to take a lot of convincing.
“The SNP will always welcome engagement with how we can strengthen relations with Northern Ireland and Ireland, but we will focus on practical and achievable ideas - not unsubstantiated vanity projects and baseless briefings which this Tory Prime Minister is all too familiar."