Senior British MEP Charles Tannock says that "mobilising" Labour voters is "essential" for a Remain victory in the upcoming referendum on the UK's membership of the EU.
Tannock was responding to a key speech on Thursday by UK Labour leader Jeremy Corbyn who said that he has concerns about the EU but believes it to be best way to tackle global challenges.
Corbyn urged Labour supporters to back the EU "warts and all" in a bid to allay fears among remain campaigners that a failure to motivate Labour voters could leave Britain at risk of crashing out of the EU.
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The party leader's long-trailed intervention in the debate over the UK's future comes as former deputy Prime Minister Nick Clegg called on Labour to issue a "clarion call" to its supporters, warning that he believed the June referendum could now be lost.
Corbyn, a long-time critic of the EU, told an audience at the University of London's Senate House that he still has concerns about how the EU works but believes it is the best way of tackling major global challenges such as climate change.
Tannock, a member of Parliament's ECR group, told this website, "I welcome Jeremy Corbyn's commitment to the EU Remain campaign.
"I would disagree with him on many things and he has probably opposed the EU in the past on the basis of it being a free market club.
"But he has made a journey and come to realise that in an interconnected world you cannot take on and engage with issues like climate change, transparency and fairness in the taxation of multinationals and managing the European migration crisis but pulling up the drawbridge and acting in isolation."
The Tory deputy went on, "I will be campaigning alongside all parties and UK citizens of goodwill who believe UK is stronger in a reformed EU for our prosperity and security and believe that mobilising Labour voters is essential for a Remain victory. I will be doing my bit to mobilise all undecided particularly in own party the Conservatives to get out and vote on June 23."
ECR MEP Sajjad Karim said, "Jeremy Corbyn finds himself in a pivotal position to help deliver a result which is in our national interest and guarantees for our coming generations a much broader canvass on which to develop themselves and the UK.
"It is a time for all those who recognise global realities to stand and fight to protect our future from an uncertain abyss wrapped in emotive "glory days" packaging - for all those who believe that our best days are ahead of us not behind."
Further reaction came from UK Liberal Democrat MEP Catherine Bearder who said, "I'm glad Jeremy Corbyn is finally speaking out about why Britain is better off inside the EU. Our prosperity, security and environment are all better protected as part of the world's largest trading bloc."
Socialist MEP Anneliese Dodds, commented, "I'm pleased Jeremy Corbyn has made such a clear and passionate case for Britain as part of the EU. Jeremy is right: we can only solve modern-day challenges like climate change, the refugee crisis and tax evasion by working with other countries"
Elsewhere, Andrew Duff, President of the Union of European Federalists, said, "It's my belief that the British have effectively already decided to leave the EU, one way or the other. It's only the detail that has to be settled: how quick and how far away they go; if they take anyone else with them; and what will they end up with: third country or associate membership status."