Jeremy Shapiro claimed the UK likes to think of itself as a diplomatic superpower, but said “those days are over”.
The foreign relations director insisted Britain now relies on the US after deciding to Leave the European Union.
Speaking on Newsnight, he said: “I think it’s probably a bit outdated. I know that Great Britain has a great history of diplomacy, I know it likes to think of itself as a great power.
“But the truth is it’s been a medium power for decades now.
Jeremy Shapiro insisted Britain is not a global superpower
“It’s punched above its weight during those decades because it has had a good relationship with the United States and because it has been a member of the European Union.
“And it’s actually managed to work both sides of that isle and punch above its weight diplomatically.”
Mr Shapiro claimed the UK needed to deal with the consequences of voting to Leave the European Union last June, as he said it reduced the country’s power.
He added: “But with the Brexit decision those days are sort of over.
“You can see that Britain needs to align itself ever more closely with the United States.
The truth is Britain has been a medium power for decades now
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“That means it has less influence with both the Untied States and with its European partners.”
The director finished by claiming Britain has been forced to side with the US, further reducing its power.
“You have nowhere to go,” he said.
Labour's Shadow Foreign Secretary Emily Thornberry came to the defence of the UK, saying Britain’s relationship with the European Union and US were “profoundly changing”.
In pictures: Theresa May meets with EU's Tusk Thu, April 6, 2017
The two leaders held talks on Brexit negotiations
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European Council President Donald Tusk gestures to members of the media as he leaves 10 Downing street after talks with British Prime Minister Theresa May in central London
She said: “I think you underplay the many positives we have as a country which means that we can continue to punch above our weight.
“We don’t have to play second fiddle to the US at the security council we could have a distinctive voice.
“Obviously we’re not a superpower. But I think we should have more influence than the size of our country.”