Scotland's transport minister has branded the UK's government's proposed bridge between Scotland and Northern Ireland a £20bn "vanity project".
Michael Matheson criticised Prime Minister Boris Johnson for not consulting Holyrood before announcing officials were looking into the idea.
Mr Matheson said the money earmarked for a new bridge would be better spent on projects "that will improve lives".
Downing Street has said the PM believes the bridge "would have some merit".
It was revealed on Monday that work was under way "by a range of government officials" to look at the idea of building the bridge.
Boris Johnson described it as a "very interesting idea", while Ireland's premier Leo Varadkar said it was "worth examining".
Two routes have previously been put forward - from Portpatrick to Larne or near Campbeltown to the Antrim coast.
The Portpatrick route would be more than 20 miles across the North Channel.
Mr Matheson has written to his UK government counterpart Grant Shapps to express his "concerns" about the proposals.
He said there had not been any discussions with the Holyrood or Stormont administrations, despite transport being a devolved issue.
Mr Matheson wrote: "I strongly believe that if £20bn is available for investment in infrastructure in Scotland and Northern Ireland that rather than indulging the prime minister with this vanity project, such funding should be made available to our respective governments so it could be better spent on meeting the priorities of the people we represent.
"I therefore request immediate discussions with your officials on releasing to us the £20bn of funding you have identified so it can be invested in the priorities of Northern Ireland and Scotland."
Mr Matheson also raised concerns about a reported "munitions dump" at Beaufort's Dyke in the North Channel, on the path of construction for the proposed bridge.
He said money spent to work around the problem "could be put to better use on practical, deliverable projects".
The Scottish minister has been backed by Northern Ireland's infrastructure minister, Nichola Mallon, who said she was "surprised and concerned" by the proposals.
She added: "I am extremely concerned that pursuit of this project, costing £20bn, will be a waste of significant money and resource that could be put to better use by addressing pressures and deliverable projects here in Northern Ireland and elsewhere."
How much could a Scotland-Northern Ireland bridge cost?
The price of any construction would, obviously, be dependent on the route chosen.
More than a decade ago the think tank the Centre for Cross Border Studies suggested a 21-mile bridge from Dumfries and Galloway could provide international rail links and ease the strain on air services.
At that time it estimated the cost of the scheme would be about £3.5bn.
However, by last year the suggested price tag had risen considerably.
Some experts have suggested £15bn might be required for the project but others have said that £20bn would be a "conservative estimate".