The Infrastructure Minister has written to Boris Johnson calling on him to give Stormont the funds for his scrapped bridge between Northern Ireland and Scotland.
Nichola Mallon said the Prime Minister's shelved investment should be redirected to public services in Northern Ireland.
It follows reports that British government proposals for a bridge or tunnel to Scotland have been ruled out due to forecasted costs and engineering challenges.
A review of connections between different parts of the UK by transport expert Sir Peter Hendy concluded the project was not currently viable, the Daily Telegraph reported.
A government source told the newspaper that Sir Peter found "it would be technically very challenging at the moment".
Ms Mallon has long dismissed the bridge or tunnel idea as a "Tory vanity project".
The SDLP deputy leader said the so-called "Boris Bridge" is "not only technically unworkable but is a gross misuse of public money".
She said: "Now this issue is set to be put to bed, I have today sent a memo to the Prime Minister calling for this promised investment to be provided to the north so it can be used to enhance our public services to ensure we can deliver what our citizens and communities deserve."
The North Belfast MLA added: "It's long past time the promises of investment and delivery in New Decade New Approach materialised."
Mr Johnson has been a vocal supporter of a fixed link between Britain and Northern Ireland, which some experts have estimated could cost £20billion.
Two potential routes for a link have been suggested – from Portpatrick to Larne, or near Campbeltown to the Antrim coast.
While some including the DUP have been supportive of the idea, others have labelled it unfeasible and too costly.
Earlier this year Mr Johnson's former chief adviser Dominic Cummings dismissed the proposal as he criticised the Prime Minister's priorities.
Mr Cummings told the BBC: "The Prime Minister's only agenda is, buy more trains, buy more buses, have more bikes, and build the world's most stupid tunnel to Ireland. That's it."
The UK government's Department for Transport said the Union Connectivity Review would be "published shortly" and it would not comment on "speculation".
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