A 150-year-old tunnel used by Dublin City Council to supply more than 100,000 homes with water could collapse at any moment, it has been revealed.
The local authority said it has identified a problem with the Vartry Tunnel, which is a critical artery for the supply of water to the Dublin and Wicklow area.
However, the council will not have the €20m needed to upgrade the structure until after 2012.
Some 80 million litres of water flows through the tunnel every day, representing 25pc of the total of the council's supply to the Greater Dublin Area.
Tom Leahy, deputy city engineer with the council, said the structure is a rock-lined tunnel laid through the Wicklow mountains in 1860.
He said the council has identified "a serious problem" which could cause it "to collapse at any moment in time".
A spokeswoman for the council told the Herald: "It is forecast that the costs to upgrade the Vartry tunnel are in the vicinity of €20m. However, this funding won't be available until after 2012."
She added that, to date, "the council has spent €40m upgrading 60km of network under the water mains rehabilitation scheme in the Dublin region".
In a Dublin City Council document on its capital projects, the local authority stated studies have indicated that the "tunnel is not stable".
"A scheme is being advanced to build a new section of tunnel to improve security of supply ... It is hoped to move this scheme to construction post 2012."
It added: "Any failure of this rock-lined tunnel would deprive over 150,000 customers of guaranteed availability of drinking water for months or years."
Mr Leahy said the current water crisis "is a wake-up call for us all". "We now know that our infrastructure is ageing. We know the problems, we know what has to be done," he told RTE's Prime Time.