The Irish FA chief executive has appealed to Northern Ireland football fans to back the Euro 2028 games at Casement Park.
In a letter sent to the Amalgamation of Official Northern Ireland Supporters' Clubs (AONISC), Patrick Nelson said “this is a pivotal moment for football and society".
Mr Nelson also ruled out any prospect of Windsor Park being expanded in time to make it an alternative venue in Northern Ireland to stage the Euro 2028 games, if the redevelopment of Casement Park is not completed in time.
Responding to a suggestion from AONISC to bring Windsor Park, the current home of the Northern Ireland team, up to the required capacity for the tournament, Mr Nelson said there is "no route to fund an expansion" of the stadium.
UEFA rubber-stamped the Ireland and Britain joint bid as Euro 2028 tournament hosts last month, with west Belfast's Casement Park the only proposed venue in the north.
The stadium still remains a concept, with work on the long-awaited £150 million 34,000-seater venue - the home of Antrim GAA - yet to begin after years of setbacks since the project was first announced.
Some Northern Ireland football fans are opposed to the GAA venue being used as part of the joint UK and Ireland hosting of the Uefa international showpiece.
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During Euro 2024 qualifiers at Windsor Park last month, a section of the home support sang a crude anti-Casement Park chant, while a banner declaring ‘No Casement’ was hung in the stadium.
In a letter sent to the Irish FA, Gary McAllister, chair of the AONISC, asked Mr Nelson, what would be “Plan B” if Casement Park was not ready in time for the Euro 2028 games and suggested an upgrade of Windsor Park.
But Mr Nelson replied: "There is no current funding opportunity from government for any extension to the National Football Stadium at Windsor Park".
Mr McAllister had also expressed concerns in his letter over funding for the local game and the legacy for the sport by playing Euro 2028 games at Casement Park.
He also raised concerns about the safety of Northern Ireland fans travelling to the venue in an area of west Belfast, which has a predominantly nationalist population.
Mr Nelson in his response said that 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," he wrote.
"As the governing body for football in Northern Ireland we place great emphasis on the role of our national team in the European and wider global game.
"For us not to be involved in the bid to host this tournament was simply not a viable option for us."
Mr Nelson also said that the "planning for the access and egress of fans to stadia (Casement Park) will be given every consideration with governmental partners and the PSNI".
"Such planning is an ongoing part of any major sporting or public event," he added.
"There is no question it will be overlooked and we will work closely and diligently on all aspects of this."
The IFA chief said that the "long-term benefits and consequences of hosting a major sporting event" should be considered.
"Given that people here are still on a journey after a difficult past, it would be a shame if we missed the opportunity for better engagement between communities," he said.
"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."