Franklin's Gardens has a capacity of 13,500
Northampton Saints have been forced to move their Heineken Cup quarter-final against Ulster to Milton Keynes.
Saints had sought dispensation to play at Franklin's Gardens, which falls below the 15,000 capacity required by the tournament organisers.
But they have now been refused and must play the game at Stadium MK in April.
"The regulation is clear and they've refused us. But we're not afraid of playing at Stadium MK," chief executive Allan Robson told BBC Sport.
"It's a matter of making it our home for the day."
In August 2010,
Saints expressed their concerns about having to move from the Gardens if they reached the knock-out stages.
"There's no appeal, so we've now got to get on," said Robson.
I still think that Northampton have an unbelievably good chance to go all the way to the final
Former Saints and England winger Jon Sleightholme
"Stadium:mk is a lovely stadium. We know very well that we normally sell out at Franklin's Gardens for what you'd call an ordinary game.
"For a quarter-final in the Heineken Cup, with 21,500 capacity, I expect to sell it out."
Jon Sleightholme played in the Saints team that won the European Cup in 2000. He does not believe the move will impact on the team's performance against Ulster.
"My preference would be to play at the Gardens," said Sleightholme, who recalls the European Cup quarter-final over Wasps in 2000.
"When we beat Wasps, playing at home was definitely an advantage. It was a tight game and the home support made a difference.
"But I don't think the playing staff will be too bothered. They'll fancy their chances wherever they play.
"There will be a phenomenal amount of Saints supporters and it will feel like home."
If Saints can advance past Ulster, they will entertain either Perpignan or Toulon in the semi-finals.
"I still think that Northampton have an unbelievably good chance to go all the way to the final," added Sleightholme.
Saints had been attempting to add to the capacity of the Gardens.
Their plan was for Asda to pay for land to build a store and that revenue would pay for the expansion, but the council have indicated this would not be in keeping with town plans.
"It makes me a bit angry," said Robson. "Everyone knows we have a wonderful stadium - it just happens only to be 13,500.
"We are looking for permission to develop one of our stands and extend the capacity to 17,500. If we had that, we wouldn't be talking about this.
"It's annoying. We are called Northampton Saints because that's where we belong - Northampton, not Milton Keynes."