An abandoned rail tunnel dubbed the "highest risk to public safety" has been declared "generally stable" by a group campaigning to save it.
Queensbury Tunnel runs 1.4 miles (2.3km) between Bradford and Halifax and is partially flooded and collapsed.
Highways England, which looks after the tunnel, said it "represents the highest risk to public safety" of its 3,200 former railway structures.
Closure would mean an "opportunity will be lost forever", said campaigners.
The Queensbury Tunnel society wants to reopen the structure as part of a strategic link in the National Cycle Network.
In a new report the group said: "a considerable sum of taxpayers' money could soon be spent putting Queensbury Tunnel permanently beyond use despite the risk to the community never having been quantified."
The tunnel was last used by trains about 60 years ago.
Graeme Bickerdike, Engineering Co-ordinator for the Queensbury Tunnel Society said the tunnel "currently presents little short-term risk to the community, as has been the case for several decades."
Highways England had not effectively assessed the tunnel's risk, he claimed.
The cycle path scheme, despite "complex issues", is a "unique opportunity to get a nationally significant piece of infrastructure", he added.
Opened 1878, closed 1956
- Took four years to build with the loss of 10 lives
- On completion it was the longest tunnel on the Great Northern Railway.
A spokesperson for Highways England said: "Unfortunately the condition of the tunnel has severely deteriorated since it was closed.
"Due to the poor and worsening condition of the tunnel it is a priority to permanently close the tunnel on the grounds of public safety."
It said it would consider transferring ownership to another public body but closure work could start in September, subject to planning.