The proposed metro line running from Shanganagh in south Co Dublin to Dublin Airport will not be finished until at least five years after the government's target date of 2007, according to a feasibility study carried out by management consultants.
The study also warns that the project will cost billions of euros more than anticipated.
It estimates that the line would cost as much as €20 billion and could not be delivered for at least 10 years.
Even a line from St Stephen's Green to the airport, which would involve tunnelling under the city centre and the River Liffey, is likely to cost almost as much and take at least eight years to deliver, according to the consultants.
According to reliable sources, these are the principal findings of the feasibility study by management consultants of the metro plan put forward two years ago by the Dublin Transportation Office (DTO).
At the time, it was estimated that all of the public transport elements in the DTO's Platform for Change policy, including more Luas lines, DART improvements and the metro, would cost €8 billion. It was envisaged that the DTO's metro plan - the key element of Platform for Change, which was accepted by the Government in July 2000 - would be financed through a public-private partnership.
But the consultants have concluded that any such PPP for the metro would cost in excess of €1 billion a year for 20 years, including construction, franchising, operation, maintenance and interest charges.
"What this means is that the metro plan is running off the rails because we can't afford it either in terms of time or money", one source told The Irish Times. "It throws the whole DTO strategy up in the air."
The feasibility study, carried out by KPMG for the Railway Procurement Agency, is to be considered by the RPA's board before being submitted to the Minister for Transport, Mr Brennan, and his Department's Metro Monitoring Committee.
It is understood that the European Investment Bank, which had been requested to consider providing some of the funding, has also expressed scepticism about the viability of the metro project.
The Agreed Programme for Government between Fianna Fáil and the Progressive Democrats said the metro would be developed on a PPP basis with a view to "achieving a link to the airport by 2007".
As envisaged, this timetable - recently reaffirmed by Mr Brennan - would be met by "making the maximum use of private finance" to cover the costs of constructing and operating the proposed metro line.
However, if the cost is as prohibitive as the consultants suggest and the line cannot physically be delivered for at least 10 years, other options to serve the airport by rail would have to be examined.
As a result, plans put forward in 1999 by a CIÉ-Aer Rianta working party to provide a rail link from Connolly Station to Dublin Airport, via a new spur off the Maynooth line, are now likely to be "dusted down".
Last night, the DTO said it stood over its original construction cost of about €2 billion for a metro line from the city centre to the airport, adding that the consultants had not taken revenue into account.
"It appears that the €20 billion figure ... does not take into account the revenue metro will generate", said the DTO's director, Mr John Henry.
Without that, "this figure can have little meaning", he added.
A spokesman for the Minister said he had "not received any report from consultants" on the metro costings, but Mr Brennan was "adamant" that a metro link to the airport should not take 10 years.
