A Mills bomb is ticking away under No 10"s Immigration Bill It is true that No 10 has form in backing down on EU-related matters. After all, it later conceded the very referendum that was the cause of that record rebellion. It also folded in the face of an amendment to the Queen's speech regretting the absence of a referendum Bill. But it is almost impossible to see how Mr Cameron could swallow Mr Mills's amendment. To withdraw unilaterally from the free movement of peoples, one of the EU's most venerable articles of faith, would arguably be to leave it altogether. This is certainly Nick Clegg's view and that of his party. Neither its MPs nor its activists would swing behind the Mills amendment - especially since, as the election draws nearer, the Liberal Democrats are busy differentiating themselves from their Coalition partner. Were Downing Street to press the matter, the Government would become unworkable. At best for him, the Prime Minister would be left leading a minority Conservative administration. At worst, given the wondrous workings of the Fixed Term Parliaments Act, the Queen might have to send for Ed Miliband. Mr Cameron is not willingly going to bring about such a pass. But even if he had more room for manoeuvre, it is very doubtful that he would open himself to the accusation of leaving the EU without a referendum - and acting illegally to boot. That these allegations are mutually exclusive doesn't seem to worry the amendment's critics. This impasse over the Mills amendment does help to explain the flurry of briefing at the weekend over the Home Office's view on restricting free movement, and the furious reaction from the Deputy Prime Minister yesterday. The debate within the Government is not the equivalent of two people peering at a map before starting a journey and planning a route together. Rather, Mr Clegg and Theresa May are already in the car, squabbling about the navigation and each making grabs at the steering wheel - all the while with no agreed destination. The Prime Minister is on the Home Secretary's side. He could scarcely not be, having floated the idea of restrictions on free movement himself late last month. But in one sense, the quarrel between the Coalition partners scarcely matters. Perhaps, under pressure from anti-immigration parties, other European governments will, in time, come down on Mr Cameron's side, and back restrictions on freedom of movement, or at least support proposals that would empower EU member states to return migrants to their countries of origin if they don't have jobs, and restrict their access to benefits if they do. But even if they take action, it won't happen by January 1. The Prime Minister's view on border control is rather like that of St Augustine's on chastity: "Give me an end to the free movement of peoples, Lord - but not yet." So what will happen in a fortnight's time? One plausible response is: very little. In 2004, only Ireland and Sweden joined Britain in opening up its borders to the Poles and others. Ten years later, every EU country will have done the same for the Romanians and Bulgarians by the start of next month. This time round, then, the flow should be more widely dispersed. But the truth is that no one knows what will take place. And, whatever the attractions of our benefit system may or may not be, Britain has a unique pull for migrants as the home of the English language, one of the most widely-spoken in the world. Migration Watch predicts that the UK population of both nationalities will grow between 30,000 and 70,000 in each of the first five years. It is scarcely xenophobic or racist to point out that this would be rather a lot of people, though there may be no firm figures by the next election with which to make a judgment. However, that may not matter very much - at least as far as voters are concerned. The first arrivals in January will excite a firework display of publicity. The press and the public will take a close interest in what is happening during the weeks that follow. As both do now: this newspaper today reports that up to 40 illegal migrants enter Britain via Calais each day. Any large movement into the country is unlikely to disperse itself evenly, and there will be particular pressure points on housing, schools, the health service and transport. There are many objections to the Mills amendment - its illegality and unworkability being two of the most substantial. Certainly, January 1 will have come and gone long before it is debated (if, indeed, that ever happens). But with the European elections looming in May, Ukip's poll ratings nudging up again, and an unknown number of Eastern Europeans due to arrive - not to mention an Immigration Bill in suspended animation - we haven't heard the last of it. Nor has the Prime Minister.