Emily Thornberry hailed the UK as an influential nation that will dominate the agenda after it divorces the European Union.
Before the Labour frontbencher launched her pro-Britain speech, Mr Shapiro, director of research at the European Council on Foreign Relations said: “Britain likes to think of itself as a great power, but the truth is its been a medium power for decades now.
Emily Thornberry hails post-Brexit Britain on BBC Newsnight
“It’s punched above its weight because of its good relationship with the United States and its been a member of the European Union.
"It's managed to work both sides of that aisle and punch above its weight, but with the Brexit decision, those days are over."
Brexit day: images from around the country as Britain invokes Article 50 Mon, April 3, 2017
The country reacts as Theresa May officially invokes Article 50, and begins the process of Britain leaving the European Union
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Pro-EU demonstrators protest outside Parliament
But Ms Thornberry promptly slammed Mr Shapiro, and said: "I think you underplay the many positives we have as a country, which means we can continue to punch above our weight – we have a place on the security council.”
Ms Thornberry even silenced Newsnight host Evan Davis who appeared to side with the EU expert.
Listing reasons why Britain will thrive post Brexit, Ms Thornberry added: “We don’t have to play second fiddle to the US at the security council we could have a distinctive voice.
"People speak English, we have soft power, we spend a large proportion on aid, a lot of world leaders were educated in Britain and we spend money on defence.”
“Obviously we’re not a superpower. But I think we should have more influence than the size of our country."
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Boris Johnson, far right, humiliated at the G7 meeting in a bid to secure UK interests
Thornberry laughed at the EU Council expert as he ridiculed Britain
Her comments on Newsnight come after Boris Johnson was humiliated at the G7 meeting.
Refusing to back the Foreign Secretary, Ms Thornberry said Mr Johnson used the "wrong tactics" in demanding Putin faces tough sanctions.