Vladimir Putin would welcome Britain quitting the European Union, Labour's shadow foreign secretary warned on Thursday.
In a strong intervention in the referendum debate, Hilary Benn said today: "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."
Benn's warning came as Donald Tusk, the president of the European Council said David Cameron's negotiations with other EU leaders over a new deal for Britain was "fragile".
In a speech at the Chatham House foreign affairs think-tank in central-London this morning, Benn said it would be an "extraordinary folly" for voters to choose Brexit.
"Our national security is served by both our membership of Nato and of the EU. To walk away from our membership and leadership of the EU would be a grave strategic error because Britain’s role in promoting international peace and security around the globe is greatly enhanced by being part of Europe," he said.
"Let us be clear, if that were to happen it would greatly diminish our standing in the world."
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Benn said he had changed his mind on Europe since the 1975 referendum - when he voted to leave. "I have been on a journey, not least because Britain has been on a journey too," he said.
Corbyn voted to leave Europe at the 1975 referendum and made eurosceptic noises during the Labour leadership campaign. However the Labour leader has since said he is in favour of EU membership.
Tusk said in a statement: I am in a process of intensive talks about my proposal for the UK settlement.
"I am confident that this is a balanced and solid proposal and I hope to finalise it next week in the European Council. However let me be clear, this is a very fragile political process."
Cameron is hoping to secure for Britain a renegotiated membership of the EU at the meeting of the Council on February 18. The prime minister is then due to deliver a statement to the House of Commons on Monday February 22 to inform MPs of the outcome.