Work to keep part of an abandoned tunnel from collapsing could end plans for Europe's longest underground cycleway, campaigners have said.
It was hoped Queensbury Tunnel could be used by cyclists travelling between Bradford and Halifax.
But a bid to reopen the 1.4 mile (2.3km) tunnel has been threatened by emergency work to fill in an air shaft.
Highways England said the work means "any reopening is now going to be more challenging".
Campaigners, who have been fighting to reopen the tunnel for more than five years, described the work as "the engineering equivalent of knocking your house down because a hole has formed in the guttering".
A spokesperson said: "It has the potential to put permanently beyond use a historic asset which could deliver long-term social, economic and tourism benefits to the region."
Highways England had been carrying out safety work to provide strengthening to sections of the tunnel, with contractors said to be close to completing the first phase.
But at the end of September, as contractors carried out a close inspection of an air shaft, water started to enter the tunnel at an "unprecedented volume and speed" forcing them to abandon equipment and move to a safe area.
Highways England said the air shaft would now be filled in from above to prevent it from collapsing.
Richard Marshall, a regional director, said: "We are aware that this news will be a disappointment to those seeking the reopening of the tunnel.
"However, we have no option other than to complete this work immediately to ensure both the safety of those communities living close by and the workforce who need to maintain it."