The warning comes in the wake of Ireland's worst ever traffic jam on the M50 and N11 last Wednesday evening.

The authority is implicitly critical of Ikea for failing to address, or provide solutions to, the road capacity deficiencies. Those deficiencies are "clearly recognised within the planning application information". That confirms that the currently approved road network, even allowing for the M50 Upgrade Scheme, is "unable to cater for the Ikea development, " the NRA says in an appeal lodged with An Bord Pleanala against Fingal County Council's decision to grant Ikea planning permission.

The M50, the country's busiest road, was the scene of a 15-mile tailback during last Wednesday's traffic gridlock on the southside of the capital and often resembles a car-park during peak hours and even on Sundays.

Fears that the proposed Ikea store could significantly add to that congestion have now received clear backing from the NRA. It says that the development "as currently proposed" is likely to cause "significant traffic delays and congestion at the Ballymun Road junction of the M50, [and] cause substantial queuing which is likely to extend back onto the M50 motorway itself and which would, upon the occurrence of such queuing, result in significant delays and congestion for traffic on the M50."

The state body bluntly states that this scenario would be "unacceptable on this section of motorway, which forms part of the most critical artery for traffic distribution around Dublin and which serves as a vital link to Dublin airport and Dublin port."

The additional information provided by Ikea confirms that with the proposed development in place, the Ballymun Road junction on the M50 . . . even after it is upgraded . . . "will be unable to cater for the predicted future traffic levels, " the NRA says.

And it adds: "Fundamentally, the applicant has "not sought to address these deficiencies or to provide solutions to these issues. Instead the Additional Information submission stated that 'it is not however considered appropriate for Ikea as a private developer to comment further or to undertake the strategic planning of the area'." It also notes these "deficiencies" have not been addressed in conditions attached to the planning permission granted by Fingal County Council.

The "core issue", the NRA says, is whether consent should be given to a development which "acknowledged within its own documentation" cannot be accommodated by the currently approved road network and junctions, "and which has in place no proposals or measures to address, if it were possible to address, those deficiencies."

The NRA goes on to express "great concern" that the Ikea proposal will "undermine the benefits" of the 1bn investment to upgrade the motorway, the construction phase of which only began last year.

"With only one third of the upgrade at construction stage, an application is being presented which, if proceeded with in its current form, would significantly undermine and negate the benefits to be delivered by the substantial investment in that project, " it said.