David Cameron has challenged Ed Miliband over whether Labour MEPs will vote against a cut in the EU Budget, amid fears the European Parliament may veto the deal.
Last week European leaders agreed to cut the EU's budget for the next seven years to 908bn euros. Cameron has claimed the unprecedented decision as a personal victory, after assembling an alliance of "like minded" countries including Germany and Denmark.
However the budget first has to be approved by MEPs. European Parliament President Martin Schulz has said he intends to deploy a rarely used procedure that would result in a secret vote.
By allowing MEPs to vote anonymously, the move will effectively stop EU leaders galvanising their own members of the parliament to support the budget deal.
Speaking in the Commons on Monday, the prime minister said Tory MEPs would vote in favour of a cut in the budet. And Lib Dem deputy leader Simon Hughes confirmed his party's MEP's would be ordered to do the same.
Cameron attacked the secret ballot as "completely wrong" and challenged Miliband as to whether his "socialist" MEPs would back the budget cut. The Labour leader, who had already used up his time to speak, was unable to respond.
"A secret ballot would be wrong we need an open ballot," Cameron said. "I would encourage every MEPs from across the UK to support the deal."
"MPs and MEPs should vote transparently so their constituents can hold them to account. They have to account not only to their electorates but to their own countries who will suffer if a deal isn't passed."
Last October eurosceptic Tory backbenchers joined with the Labour party in a Commons vote to demand the prime minster seek a real-terms cut. Cameron had initially urged his party to support his vision of a freeze in the budget rather than a cut.