Britain's vote to leave the European Union will lead to the political end of the United Kingdom, the president of the Council on Foreign Relations told CNBC on Friday.
There will "no longer be Great Britain, at best you'll have Little Britain," Richard Haass said on "Squawk Box."
"This is Dissolution Day," he said. "This will light the fuse for Scotland, a second referendum [to leave the U.K.]. They'll opt for Europe over the U.K. I [also] worry about then what happens in Northern Ireland."
Northern Ireland's deputy leader Martin McGuinness on Friday called for a vote to unite the two sides of the Irish border. Roman Catholics in British-ruled Northern Ireland have long sought to reunite with Ireland. Under the EU, the border issue was diminished. McGuinness, a senior member of the Sinn Fein party, was formerly head of the militant Irish Republican Army.
As for the Scots, they voted two years ago to remain in the U.K., though a key argument at the time was a desire to stay in the EU.
"It's easy to sound melodramatic, but the European project is three quarters of a century old, and people take its peace and prosperity for granted," said Haass, an ambassador during the presidency of George W. Bush and an advisor to George H.W. Bush.
There's too much emphasis on the negatives of the EU such as the inefficiencies and over-regulation, Haass said. "But this is playing with history, and people are going to look back on this and they're going to say, 'This was a colossal mistake.'"
The Brexit vote shows the "danger of governing by referendum," he argued. "People went to the polls without the full understanding of the consequences of what they were setting in motion."
The "knock-on effect" of other nations looking to leave the EU is "very real," Haass said. "It goes far beyond whatever gyrations happen in the market this week."
Far-right movements in several European nations will feel emboldened by Britain's vote, he argued. For example, he added: "You'll have people in the right in France calling for Frexit."
The European Union, founded in 1951, has 28 member nations, including the U.K., whose exit from the group will need to be negotiated over course of what's expected to be at least two years.
— Reuters contributed to this report.