LibDem sources said that Labour should "step up and make the case for Britain remaining in Europe".
The charge came after Labour adopted a similar policy on holding an in/out referendum on the UK's EU membership to the LibDems, potentially making any future Coalition negotiations between the two parties easier.
Labour also announced that their proposal would be included in their 2015 General Election manifesto.
Within hours, the LibDems had announced the same.
However, aides to Nick Clegg refused to be drawn on whether the commitment would feature on the front page of their manifesto. The LibDem leader has told voters that is where to find policies his party will be prepared to argue heavily for in Coalition talks.
Labour was last night accused of a fudge on the vote.
Critics said that the party was trying to have its cake and eat it -reassure business it had no intention of holding a vote but telling voters it might.
Labour backbencher MP Graham Stringer called the move as a "shoddy compromise".
"I think the public are clear that they want a referendum. This is so ambiguous as to be impossible to sell on the doorstep," he said.
In a speech at the London Business School, Mr Miliband said he would legislate to trigger an in/ out referendum on the EU if there was an attempt to transfer powers to Brussels. He also made clear that he did not see such a scenario taking place in coming years.
Tory leader David Cameron has already pledged to give voters an in/out referendum on the EU.
The Prime Minister has said he will renegotiate the UK's position within the EU before then. Voters will be asked to make their decision on this "new deal" with Europe.
But Labour insists that a 2017 deadline is bad for business and creates economic uncertainty.
All parties are under pressure to respond to the growing electoral potential of Ukip in parts of England.
Last night Mr Miliband insisted that Britain's future "lies in the EU".
Millionaire Labour donor John Mills, the JML home shopping magnate and chairman of Labour for a Referendum, said the public would feel "short-changed" if they were not given a vote.
But CBI president Sir Mike Rake said: "Business will welcome Labour's decision to make its policy position on Europe clear. Any uncertainty is unhelpful when trying to secure long-term investment."
The Conservatives said that Mr Miliband's plan made "no sense whatsoever".