Campaigners opposed to two liquefied natural gas (LNG) terminals in west Wales must wait to see if a bid for a judicial review has succeeded.
The terminals at Milford Haven will receive LNG - gas cooled to liquid and carried by tanker.
London's Appeal Court heard lawyers for protestors claim an "inflammable cloud" could engulf the area in a disaster.
But supporters of the project have said it is safe. Three Appeal Court judges reserved judgement to a later date.
The project will see LNG imported to Pembrokeshire from across the world by tanker.
LNG will be treated and converted back into its natural state at the two terminals currently under construction. It will then be transported by pipeline into the UK gas network.
The ships will begin to arrive by the end of 2007, and up to £4bn worth of gas is due to be fed into the UK's supply over 15 years.
Safety fears have already been expressed by some local people over the possible consequences of a shipping accident or leak, although Milford Haven Port Authority has assured the community it has nothing to fear.
In July 2005, The High Court in London rejected an application by members of the Safe Haven campaign group for a full judicial review into the planning consent for the scheme.
'Risk of death'
The protesters' fight moved to the Court of Appeal, where their attempt to overturn the earlier judgement was heard on Friday.
Robin Purchas QC, representing residents, told the three judges that if disaster struck at one of the proposed new terminals, 20,000 people could be exposed to the risk of death from "a highly inflammable cloud several miles long".
Mr Purchas said construction of the giant terminals would violate their "right to life" and to respect for their families and homes, both enshrined in the European Convention on Human Rights.
Under challenge is Pembrokeshire County Council's decision to grant hazardous substance consents (HSCs) and planning permission for the two terminals - four miles apart at South Hook and Waterston.
Also under challenge is the Pembrokeshire Coast National Park Authority's decision to grant consents for the South Hook terminal, which lies within its administrative area.
Mr Purchas claimed HSCs had been granted for the terminals without proper consideration, "much less open consideration", of the marine risks and the threat to lives and environmental issues.
Tim Straker QC, for the council and park authority, said the High Court had been right to dismiss the judicial review challenge last year.
He said arguments that there had been inadequate consideration of marine risks were "completely wrong" and, in fact, a "considerable number of assessments" had been carried out.
Following a day of legal debate, the judges reserved their decision on the case until a later, unspecified, date.
Gordon Main, spokesman for the Safe Haven group, said after the hearing: "We feel the hearing today went relatively well. We were cheered to see the focus was on the serious issue of marine safety."
Milford Haven Port Authority said in a statement: "The safety aspect of the proposed LNG terminals will continue to be considered in detail.
"It is inappropriate to comment on the case today until we know the results of the judgement."