Feasibility study for bridge or tunnel between Scotland and Northern Ireland
The UK government says it will provide £20 million of funding to look into potential transport improvements including to the West Coast Mainline, and the A75 between Gretna and Stranraer.
Two experts have also been asked to look into the feasibility, cost and timescale of a tunnel or bridge that could link South West Scotland with Northern Ireland.
The Prime Minister Boris Johnson said he wanted to "build back better" after the coronavirus crisis in a way that brings "every corner of the UK closer together".
He will launch a consultation this spring on reforming air passenger duty - a tax on passenger flights from UK airports - in a bid to improve transport connecting all four nations.
The money will be spent on exploring the development of projects, including improved rail connectivity between the north coast of Wales and England; upgrading the A75 between Gretna, Dumfries and Stranraer; faster rail links from England to Scotland and rail improvements in south-east Wales.
But the Transport Salaried Staffs Association (TSSA) said that cutting domestic flight duty "flies in the face of the Government's climate commitments".
General secretary Manuel Cortes urged the Government to invest in "truly green public transport", such as rail, which is the "most effective intercity connection taking people to the heart of our towns and cities".
It comes as an interim report by Sir Peter Hendy, the boss of Network Rail who is conducting a review of union connectivity, was published assessing ways transport can better connect all parts of the UK.
The report set out how a UK Strategic Transport Network would deliver the ambition - upgrading direct transport links, reducing delays and stimulating growth across the four nations.
In the document, Sir Peter said he has asked two experts to lead a "discrete piece of work" to assess the feasibility of a bridge or tunnel between Northern Ireland and the British mainland.
The Prime Minister has mooted the idea of a bridge several times, and former HS2 Ltd chairman Douglas Oakervee and Gordon Masterton have been tasked with leading the technical review into such a link.
Mr Johnson said: "It's now time to build back better in a way which brings every corner of the UK closer together.
"We will harness the incredible power of infrastructure to level up parts of our country that have too long been left off the transport map.
"This pioneering review by Sir Peter Hendy gives us the tools we need to deliver on our ambitions for a UK-wide transport network that encompasses sea, rail, and road - and I also want to cut passenger duty on domestic flights so we can support connectivity across the country."
Transport Secretary Grant Shapps added: "As we build back better from Covid, it is more important than ever that we level-up every corner of our great country.
"Quality transport infrastructure is key to achieving that, which is why we are committed to boosting connectivity and bringing communities across the UK even closer together."