The Government is looking for ways to toughen the rules on new workers arriving from those countries to calm public fears of a huge new wave of immigration.

But it is concerned that, if restrictions are too tight, they will drive new arrivals into the black market.

Critics say that, in any case, the Government’s choice is almost meaningless as rules will not apply to self-employed workers and it will be hard to crack down on those working in the black market.

Despite Britain’s generally sunny experience of immigration from Poland, there have been great concerns about the consequences of Romania and Bulgaria joining the EU.

The European Commission wrestled painfully with the decision about whether to admit them, issuing a series of searing reports about their failure to reform their economies and their courts.

Brussels has accused both countries — but Bulgaria most of all — of failing to make any inroads into a deep-seated culture of corruption and criminality.

Given their comparative poverty, there is also a chance that the level of migration may be very high, although the Home Office is understandably wary of making predictions. It reckoned that tens of thousands of Poles would arrive when Poland joined the EU two years ago; instead, more than a quarter of a million have registered for work and many more arrived as visitors or selfemployed.

The Romanian Government has dismissed predictions of huge migration, saying that it expects a lot of traffic both ways, but many are sceptical, at least in the early years.

Once Romania and Bulgaria join the EU, other member countries must drop visa requirements. Romanians and Bulgarians will be free to travel to Britain and those who declare themselves to be self-employed may start work.

Others who want to work are required, at the least, to register for work, to pay tax and formally to enter the economy. But the Government is considering whether it may want to bring in extra controls. Among the options are to set quotas for the number of work permits, or even to set quotas by sector.

But the fear is that the more restrictive and complicated the rules, the more workers will be driven into the black economy.

Ministers have been taking an increasingly tough line. Liam Byrne, the Immigration Minister, said last month of Romania and Bulgaria: “Here we think, actually, that there is a case for only very gradual access and we will be thinking through the sorts of controls that are needed before reporting back to Parliament before the end of October.”

John Reid, the Home Secretary, said that immigration from Romania and Bulgaria needed to be “managed carefully” and that “we need to ensure that we have all the necessary safeguards in place, right down to local borough level, to reassure the public that this movement will be managed fairly and competently . . . I know that law enforcement agencies have already been working closely with the Romanian and Bulgarian governments .”

Even though Britain’s experience of Polish migration — “immigrants from heaven” as they are sometimes dubbed — has been so generally positive, the Government failed to foresee the scale of it. Ministers are trying not to make the same mistake again, but do not have much room for manoeuvre.

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