The feasibility of building a bridge or tunnel between Scotland and Northern Ireland will be examined as part of a national review of transport connections across the UK.
It will look at the "cost, practicality and demand for a new fixed link".
Downing Street said the study will also consider boosting road and rail access to Scotland from the rest of the UK.
However Scotland's Transport Secretary Michael Matheson raised concerns that the study undermined devolution.
He said the review had been organised with "virtually no consultation" with the Scottish government, despite transport being devolved to Holyrood.
Mr Matheson has previously said the bridge plan was a £20bn "vanity project" and the money could be better spent on other projects.
Prime Minister Boris Johnson has in the past described the prospect of link across the Irish Sea as a "very interesting idea".
Launching the latest review, he said new transport projects could help the UK recover from the Covid pandemic and improve access to "jobs and opportunities" across the country.
Two routes have been suggested for the link with Northern Ireland - from Portpatrick to Larne, or near Campbeltown to the Antrim coast.
Any link between Portpatrick and Larne would have to overcome the challenge of Beaufort's Dyke - a deep submarine trench where an estimated one million tonnes of weapons have been jettisoned.
A bridge from Campbeltown would be cheaper, but transport links to the area are poor.
The independent review is intended to look at ways to improve connections and create new ones across Scotland, Wales, Northern Ireland and England.
It will be led by Sir Peter Hendy, the chairman of Network Rail.
Downing Street said it would provide "independent, honest advice" on improving transport links, while considering their cost feasibility and value for money.
The UK government has also asked Sir Peter to consider likely future transport need and technological developments over the next 20-30 years.
In addition to the fixed link between Scotland and Northern Ireland and improving road and rail links to Scotland, it will look at:
- Reviewing air links within the UK
- Cutting journey times to North Wales by reviewing the Welsh railway network
- Improving major road links across the country, like the A1
The prime minister said: "Quality transport links are the key to making sure everyone can access education, jobs and housing, helping businesses to grow and thrive and rebalancing opportunity fairly across our country.
"As we build back better from the pandemic, Sir Peter's review will help make sure we have the right connections to support, sustain and strengthen our communities - to truly level up across the UK."
The Scottish government said it wanted to see improved transport links but warned about the effect of the review on devolution.
Mr Matheson said: "There is a real possibility that the Tories spending money on their own priorities in Scotland will see money being salami sliced from Scotland's budget - that is completely unacceptable both to me and no doubt the vast majority of people in Scotland.
"While I will seek to engage constructively with the UK government, I will absolutely not sign up to anything which in any way undermines the devolution settlement. The ball is in the UK government's court to prove that this is not the case - but thus far I frankly have little confidence in this process."
Sir Peter's formal recommendations are expected to be published in summer 2021.