The Rhondda Tunnel Society wants the Blaencwm tunnel, in Rhondda Cynon Taff, to Blaengwynfi, in Neath Port Talbot, to be used by cyclists and walkers.
But Highways England, which currently maintains it, said the Welsh government had yet to request its transfer.
The Welsh government said it must first consider the tunnel's "liabilities".
A feasibility study into the re-opening of the tunnel was commissioned by the Welsh government last year.
But Highways England said it had "made it clear" the two-mile (3km) tunnel's ownership needed to be "formally transferred" to a statutory body, such as the Welsh government, before the project could go ahead.
"As yet, there has been no such approach," it said, in a letter to Plaid Cymru leader Leanne Wood.
Steve Mackey, chairman of the Rhondda Tunnel Society, said: "This is dreadful as the society has moved mountains in the last 16 months to be confronted by this shambles."
The group would like to turn the route - Wales' longest disused tunnel - into an attraction, with hotels, bike hire, cafes, a museum and a restaurant.
Ms Wood, who is a member of the society, said she was "incensed" the project had lost momentum.
The Welsh government said it was working with Highways England to establish the implications of any transfer.
A spokeswoman said: "Before proceeding with a transfer of the Rhondda Tunnel into our ownership it is important we first consider the legal, financial, safety and environmental liabilities associated with such a move."
The Rhondda Tunnel, designed by Sydney William Yockney, was completed in 1890. It was closed in 1968 due to the cost of required repairs.
Campaigners working to reopen a historical railway tunnel to tourists have criticised the Welsh government for delays in the project.