Tory MPs are busy hailing what they regard as a late Christmas present: the news that the UK is forecast to become Europe's largest economy by 2030. According to the Centre for Economic and Business Research (CEBR), Britain will overtake France by 2018 and Germany within the next two decades, leaving it as the second biggest western economy after the US.

The response from the Conservatives could be summarised as "See? We told you George knows what he's doing!" But here's one point they're conveniently avoiding: Britain won't win the growth race unless it maintains a high rate of immigration. As the CEBR states, "positive demographics with continuing immigration" is the main factor (along with non-membership of the euro) behind the UK's projected success. While Germany's population is forecast to decline sharply over the next few decades, the UK's is expected to rise to 75 million by 2043, making it the biggest country in Europe.

That a significant part of this increase is expected to come through immigration helps explain why Britain will grow strongly. An OECD report last month, for instance, found that migrants make a net contribution of 1.02 per cent of GDP or £16.3bn, since they are younger and more economically active than the population in general. Far from being "benefit tourists", migrants contribute far more in taxes than they receive in welfare payments and public services. Of the 5.5 million people claiming working age benefits in February 2011, just 371,000 (6.4 per cent) were foreign nationals when they first arrived in the UK, meaning only 6.6 per cent of those born abroad receive benefits, compared to 16.6 per cent of UK nationals.

Were immigration to be cut to the level most Tories would like to see (little or none), growth would be dramatically reduced. According to NIESR, a halving of net migration over the period to 2060 would shrink GDP by 11 per cent and GDP per person by 2.7 per cent. This leaves the Tories with two options: they can either welcome immigrants as contributors to the economy, or they can turn them away and accept growth will suffer as a result. What they can't do (at least if they wish to retain any credibility), is to boast of our projected growth while promoting policies that would strangle it.