Russian president Vladimir Putin would see Brexit as a sign of “weakness”, shadow foreign secretary Hilary Benn has warned.
Mr Benn highlighted the global political dangers as he insisted the case for staying in the EU is “stronger than ever”.
Delivering a speech at the Chatham House think-tank in London, Mr Benn said leaving would mean the UK was less able to deal with international challenges such as the migrant crisis and climate change.
Campaigners for Scottish independence would also take the opportunity to “have another go” at breaking up the union.
Referring to Russian action in the Crimea and Ukraine, Mr Benn said it was down to “Europe’s collective response that we have been able to exert real pressure and have an impact”.
“Efforts towards the creation of an EU-wide energy union will, over time, weaken Russia’s dominance as an energy supplier in Europe,” he added.
“Let’s be clear. President Putin would shed no tears if Britain left the European Union. He would see Brexit as a sign of our weakness and of the weakness of European solidarity at the very moment when we need to maintain our collective strength.”
The potential break-up of the UK in the wake of Brexit that would also “greatly diminish our standing in the world”, he added.
“There are voices in Scotland who would use any opportunity arising out of this referendum to have another go at breaking up the United Kingdom. Let us be clear, if that were to happen it would greatly diminish our standing in the world,” he said.
Mr Benn – who campaigned against British membership at the time of the last public vote in 1975 – said it had become clear over the subsequent decades that it was in fact very much in the national interest.
Mocking Eurosceptics for seeking to “close the doors and wish that the rest of the world would go away”, he said: “I have changed my view on Europe since 1975.
“I have been on a journey, not least because Britain has been on a journey too. We live in a changing world and if you look at the future challenges we face I believe the case for Europe is stronger now than ever.
“The story of Britain over the last century is one of a nation at the heart of world affairs. It is the story of a country that has been at its best when we have been outward looking and confident.
“In the 20th century we helped build the institutions that have given us the chance to make progress: the UN, EU, Nato.
“In the 21st century we cannot afford to reduce our influence or to isolate ourselves or shut the curtains and close the doors and wish that the rest of the world would go away.”
Questions have been raised over Labour leader Jeremy Corbyn’s commitment to fight for Britain’s EU membership, after he left the door open to supporting withdrawal during the party leadership campaign.
Mr Corbyn has expressed particular concerns about the proposed transatlantic trade deal, known as TTIP.
But Mr Benn said: “Jeremy is fully in support of Britain remaining in the EU.”
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