A deal on Brexit at this week's EU Summit rests solely on the issue of the Irish border, a former Labour cabinet minister said.
Douglas Alexander told the BBC's Inside Business programme that the issue would affect the entire population of Europe.
A crunch meeting of the European Council will take place in Brussels later this week.
For many the failure to reach agreement to date had led them to question how they were governed, Mr Alexander said.
The former International Development Secretary told BBC Inside Business.
"For Theresa May, the road to Brussels runs through Belfast," Mr Alexander said.
"In that sense, what happens about the Northern Irish backstop is, and will continue to be, absolutely fundamental to not just the interests of people here in Northern Ireland, but actually people right across the United Kingdom and people right across Europe.
'Moment of truth'
"The Northern Ireland backstop is 'the' key issue in deciding whether we get an agreement at the European Council later this week," he added.
EU leaders meet in Brussels next week for what European Council President Donald Tusk has called the "moment of truth for Brexit negotiations".
They are not expected to reach agreement with the UK next week but say they want to see if "decisive progress" has been made to convene a special summit in November, to finalise a deal.