A campaign group has cast further doubt on the idea of a 'Boris Bridge' or 'Burrow' connecting Northern Ireland to Scotland.
The UK and Ireland Nuclear Free Local Authorities (NFLA) said it would be a "real challenge" to develop either project, particularly if built in an area of the Irish Sea called Beaufort's Dyke, where tonnes of surplus munitions from two world wars and radioactive waste are dumped.
A 21-mile bridge from here to Scotland was proposed by Boris Johnson in 2018, with a price tag of around £20bn - and was widely derided at the time.
But with the UK leaving the European Union in January, discussions on how to keep the union together have gathered pace.
Costing about half the price of a 'Boris Bridge', rail industry leaders have mooted building a tunnel from Larne to Stranraer.
The High Speed Rail Group (HSRG) proposed the plans in its submission to a Government review led by Sir Peter Hendy and an interim report is expected in the coming weeks.
But the NFLA has voiced fresh concerns, referring to a recent report that noted Beaufort's Dyke is a busy "choke point" in the Irish Sea, with ferries, commercial fishing trawlers and oil tankers passing through it frequently.
They said some nuclear waste and munitions in the area may have been "short dumped", meaning there is uncertainty as to the volume and type of munitions and waste deposited, which could lie outside the dyke.
Explosions caused by degrading munitions occur "relatively frequently", it said. According to the group, at least one of those explosions generated force equal to 5.5 tonnes of TNT.
SDLP councillor Karen McKevitt, the NFLA All-Ireland Sustainable Energy Forum co-chair, said: "NFLA has reservations with any development of a bridge or rail tunnel between Northern Ireland and Scotland due to the amount of hazardous chemicals, munitions and radioactive waste that was dumped for decades in the Beaufort's Dyke area between both nations.
"Such dumping should never have taken place, but it was commonplace in how we dealt with radioactive materials in the past. As such it makes it particularly difficult to create a transport link now and we express our caution over this project."
NFLA Scotland Forum Convener, SNP Councillor Feargal Dalton, added: "Whilst new connections between Scotland and Ireland are welcome, they have to take account the considerable health and safety issues that exist within the Beaufort's Dyke area.
"A dumping ground for decades, it is also an area of considerable activity for UK nuclear submarines. There have already been a number of 'near misses' between such submarines and commercial shipping; as well as snagging the nets of many small fishing vessels.
"Is it really feasible to develop a bridge or a tunnel in such circumstances? We are sceptical and would argue the budget for it would have to be very large indeed."
NFLA said Beaufort's Dyke is used for UK nuclear submarine training exercises, noting there is "major risk of conflict" between using the area for such exercises and its heavy use by fishing vessels and oil, gas and container cargo ships.
Last summer it emerged that a nuclear-powered Royal Navy submarine had a near-miss with a Belfast to Cairnryan passenger ferry in the area.
Regarding the potential of a tunnel, report author Tim Deere-Jones said it should not be compared to the Channel Tunnel, which is 75 metres below the seabed and 115m below sea level.
The sea bed canyon achieves maximum depths of around 300m, so the tunnel would have to be at least 375m below sea level. Engineering works for such a project would be "considerably more costly and technically difficult [than the Channel Tunnel]", Mr Deer-Jones said. The proximity of a tunnel to explosions in the area "is also a matter of considerable concern", he added.
A link to Scotland has been welcomed by DUP MP Ian Paisley, who said: "I think this would be very beneficial economically and a huge infrastructural opportunity."