Nick Clegg's declaration at PMQs  that it is now a question of "when, not if" an EU referendum will be held was a significant advancement on his previous position. The Deputy Prime Minister has long supported the coalition's "referendum lock", under which a vote is triggered whenever there is a transfer of powers to Brussels, but this is the first time that he has suggested that one will be held at some point in the next three-four years.
It remains unclear whether Clegg believes this would be a yes/no referendum on a new treaty or an in/out vote on EU membership. The referendum lock, introduced through the government's European Union Bill , suggests the former but the Lib Dems' 2010 manifesto, which said that an in/out referendum should be held "the next time a British government signs up for fundamental change in the relationship between the UK and the EU", suggests the latter. At PMQs, Clegg misleadingly conflated the two, stating: "That's what we had in our last manifesto and that's what we have now acted on in government by passing legislation together in the coalition just two years ago."
But this ambiguity is less important than the fact that he now believes some kind of referendum is inevitable. One question that follows is how Labour will respond. In an interview in January, Ed Miliband explicitly stated that he would not repeal  the coalition's referendum lock ("there is legislation on the books that we don't intend repealing," he said) but has yet to say whether or not he believes a vote will or should be held in the next four years. Clegg's move means it will now be harder for him to avoid answering this question.