The Irish FA says there is "no route to fund an expansion" of Windsor Park to make it an alternative venue for Euro 2028 - if the redevelopment of Casement Park is not completed in time.
The IFA responded to a suggestion from the Amalgamation of Official Northern Ireland Supporters' Clubs to bring Windsor Park up to the required capacity for Euro 2028 games.
Casement Park, a gaelic games ground, will be NI's host venue with a capacity of 34,500 but there are concerns if it will be ready for the tournament.
Windsor Park is the home of the Northern Ireland team, but its all-seater capacity of 18,500 is well below the minimum 30,000 requirement for the tournament finals.
Some Northern Ireland fans had critical banners of the Casement Park plan at last month's match against San Marino at Windsor Park, while there were also a number of chants.
The redevelopment of Casement Park has been hindered by a number of setbacks since first being suggested about a decade ago, including long-running legal challenges.
In a letter to the IFA, the Amalgamation of Official Northern Ireland Supporters' Clubs noted its concerns and asked "What is the 'plan B' if Casement Park is not completed within the required timescale?
"With this in mind, has there been any proper exploration of the potential for the national stadium [Windsor Park] to receive investment to allow some increase in capacity, with the potential for further expansion by way of temporary seating?"
The group also expressed concerns over funding for the local game, the legacy for the sport by playing Euro 2028 games at Casement Park and the safety of Northern Ireland fans travelling to the venue in an area of west Belfast which has a predominantly nationalist population.
But the IFA CEO Patrick Nelson responded: "There is no current funding opportunity from government for any extension to the National Football Stadium at Windsor Park.
"The national stadium at Windsor Park is our home and a place we are proud of, but there is no route to fund an expansion.
"The alternative to the route we have chosen was not to have any ambition for Northern Ireland football and decline to be part of the bid. The matter of the funding for Casement Park is a consideration for government.
"The planning for the access and egress of fans to stadia [Casement Park] will be given every consideration with governmental partners and the PSNI. There is no question it will be overlooked and we will work closely and diligently on all aspects of this.
"This is a pivotal moment for football and society in this country. The Irish FA and the Amalgamation can be crucial in playing a positive role in this."