For centuries, Britain has punched well above its weight on the international stage: It built a global empire, beat back the Nazi tide and stood shoulder to shoulder with the United States during the Cold War.

But now that Britain has stunned the world with its decision to exit the European Union, experts say it will be focused inward for the foreseeable future.

"I don't think there will be the capacity or the infrastructure to look outward in the next five years," said Ian Kearns, director of the London-based European Leadership Network. "With all our diplomatic resources focused on extracting concessions from the EU, we won't be in anything other than reactive mode on other issues."

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That reality could bring a significantly diminished role on the great challenges facing the West, including Russia, Isis (Islamic State), refugees and climate change.

For Washington, Britain's distraction will be acutely felt. Britain has long been America's closest ally, one that broadly shares American interests and values, and has always formed a crucial bridge across the Atlantic.

The US looked to Britain when it needed to influence European decision-making. The EU turned to Britain when it hoped to influence the US.

Now, the loss of Britain's voice in efforts to present a united European and American front on issues such as sanctions against Russia is particularly concerning to US officials, said Philip Gordon, a former assistant secretary of state for European affairs in the Obama Administration.

Britain has voted clearly in favour of leaving the EU. Business Editor at Large Liam Dann speaks to Labour MP David Shearer and Auckland University Professor Jennifer Curtin about what happens next.

"That voice will no longer be there when withdrawal is complete," Gordon said. Instead, Britain will be preoccupied with its "great domestic convulsion". A victory for the "Remain" camp in the referendum vote was supposed to be a turning point: With Prime Minister David Cameron having put the country's two great existential dilemmas behind it - Scottish secession and EU membership - he would have a free hand to reassert Britain's role as a global power.

But Cameron's gamble of calling a referendum went badly wrong, and now Great Britain faces the very real prospect of being transformed into Little England.

Scotland is once again pressing the case for secession. Pro-Brexit voters are demanding that the country shut the door to large numbers of immigrants. And the country's political leadership and diplomatic corps are likely to spend years conducting divorce proceedings with soon-to-be former partners in the EU.

"It's taken a step to withdraw from the world," said Tom Donilon, former national security adviser to President Barack Obama. "It will go from being an important participant in the European decision-making mechanism to being a party on the other side of the negotiation table."

Britain's Prime Minister David Cameron, accompanied by his wife, Samantha, speaks to the media in front of 10 Downing street. Photo / AP
Britain's Prime Minister David Cameron, accompanied by his wife, Samantha, speaks to the media in front of 10 Downing street. Photo / AP

Donilon said that for as long as most people can remember, Britain has been the first call made by a US president when there is a challenge in the world. But the vote means calls between the White House and 10 Downing Street could matter less. "They'll remain an important player in Nato and an ally of the US. But they've diminished their leverage in Europe. Their membership in the European Union amplifies British influence. They've pulled back from that. I don't think that is good for Britain. I don't think it's good for the United States."

British politicians who campaigned for Brexit dispute that logic, arguing that a Britain freed from the shackles of the EU will be better able to assert its interests around the globe. Britain's EU membership, they have said, diluted the country's voice rather than amplified it, and actually inhibited its ability to develop its own relations with great powers like the US, as well as rising ones like China and India.

"I want to reassure everyone Britain will continue to be a great European power, leading discussions on defence and foreign policy and the work that goes on to make our world safer," Brexit campaigner and former London Mayor Boris Johnson said. "But there is simply no need in the 21st century to be part of a federal government in Brussels that is imitated nowhere else on Earth."

But foreign policy experts on both sides of the Atlantic dispute that logic, at least in the short and medium term.

The task of disentangling Britain from Europe will be gargantuan, given the extraordinary links across the English Channel in commerce, security and dozens of other areas. Britain will not only need to negotiate its way out of the 28-member bloc, it also must ink new trade deals and other agreements to replace the EU ones it has relied on to do business with the rest of the world. The process probably will take up the rest of this decade and could reach well into the next one.

Britain also faces a threat to its very existence: Leaders in pro-EU Scotland said that they will push for a new independence referendum. Yesterday, First Minister Nicola Sturgeon went further and said she would effectively circumvent London's authority over foreign affairs by reaching out to Brussels directly to discuss "possible options to protect Scotland's place in the EU". A Scottish departure would even further limit London's international clout, not least because Scotland is home to Britain's nuclear arsenal.

Britain could also be struggling economically in the years to come. The British pound plummeted and markets tumbled worldwide. Analysts say that's just a taste of the pain to come for Britain, with a downgraded credit rating, job losses, higher interest rates and a fall back into recession all potentially on the horizon.

Whoever replaces Cameron will be hard-pressed to maintain the country's current spending on defence, diplomacy and international aid.

First Minister Nicola Sturgeon, centre left, and Deputy First Minister John Swinney chair an emergency cabinet meeting at Bute House in Edinburgh. Photo / AP
First Minister Nicola Sturgeon, centre left, and Deputy First Minister John Swinney chair an emergency cabinet meeting at Bute House in Edinburgh. Photo / AP

And if Britain thinks it will at least be free of having to worry about what's happening in Brussels, it's mistaken, said Robin Niblett, director of the London-based think-tank Chatham House.

The country will still have to coordinate closely with its erstwhile EU partners, Niblett said, because "Britain is so tightly interlinked with Europe in terms of the problems we share - Russia, Isis, refugees."

The only difference, he said, is that coordination will be more difficult and time-consuming because Britain will be "on the outside rather than having a seat at the table".

Niblett said: "The irony is that we're going to spend more time focused on Europe over the next 10 years, not less."

Brexit regrexit

Courtesy of Google, we know many Britons searched for more information about what the EU is . . . hours after voting concluded. On Twitter, a hashtag circulated, #regrexit, highlighting how some people decided they had made a mistake voting to leave.

But perhaps the best evidence that people who voted for Brexit will come to regret it came via a report from the Centre for European Reform.

Polling showed the areas that had the most to lose and the least to gain from Brexit are precisely those where the referendum saw the most support. The biggest economic effect is felt by regions whose economies depend on industries such as manufacturing, mining, agriculture and utilities. These sectors participate in a lot of trade within the EU market, as opposed to domestically or globally. As one of the world's financial centres, most of London's trade occurs globally, outside of the EU.