A £1.3bn tidal lagoon in Swansea Bay could "kick start" coastal regeneration in Wales, leading assembly members have told a UK government minister.
Plaid Cymru's Simon Thomas and Conservative Russell George urged Business Secretary Greg Clark to back the green energy project.
Mr Thomas said they were given a "fair hearing" at the meeting in London.
The UK government said it was still considering the findings of the Hendry review, which backed the plans.
Mr Thomas, who chairs the assembly's finance committee and Mr George, chairman of the economy committee, stressed the cross-party support in Wales for the plan, and the positive tone of the report published in January by former UK energy minister Charles Hendry.
"This would be an important pathfinder project which could lead to a whole new industry using the expertise devised in Wales," Mr Thomas said before the meeting.
"It would be an extremely poor message to those involved in the Swansea Bay City Deal region if we lose a huge renewable investment project in the tidal lagoon due to dither and delay.
"The proposed Swansea Bay Tidal Lagoon could kick start the regeneration of our coastal communities and give a boost to our industrial heartlands."
Mr George said the delegation showed a "united front" on the issue from political parties in Wales.
Labour AM Huw Irranca-Davies - who chairs the assembly's constitutional committee - had also hoped to meet Mr Clark but was not available on Thursday.
Following the meeting, the Plaid AM said they had been given a "fair hearing" by Mr Clark.
"It is evident that the UK Secretary for State is serious about moving towards cutting carbon emissions. For individual projects we understand the Westminster Government is looking at all the figures.
"It is important as assembly members we made the case for this investment for Wales and the meeting has ensured the Westminster Government has listened to the positive messages we put forward."
A spokesman for the Department for Business, Energy and Industrial Strategy said: "The government is considering the recommendations of the Hendry Review and taking the time needed to determine what is in the best interests of UK energy consumers and taxpayers in the long term.
"We will publish our response in due course."