Just occasionally a series of events crystallises public opinion and transforms the political landscape. That is what has happened with immigration in the past fortnight. Three events stand out.

The crucial wake-up call was publication of the government’s latest population forecasts. They were truly shocking. They showed that, if immigration continues at the level the government now assumes, the population of the UK will grow by more than 10m in the next 25 years - that is equivalent to 10 cities the size of Birmingham; 70% of the increase will be due to immigration.

The public were taken aback by these numbers. They are now beginning to realise that we face the most critical decision for a generation. Not since the referendum on the Common Market in the 1970s have we confronted a decision that will so greatly affect the lives of our children and grandchildren. Do we set about a massive building programme, constructing a virtual Birmingham every 2½ years and do we accept the fundamental changes to our society that will flow from immigration on this scale? Or do we take action now to cut immigration sharply?

Next was David Cameron’s decision to speak about immigration for the first time in two years. His speech on Monday called for a “grown-up” debate, set out the dilemma in measured terms and outlined Conservative proposals for both an annual limit to immigration and, importantly, a substantial reduction in numbers.

He clearly struck a chord. It was not long before we had Labour and Conservatives competing to sound tough on immigration - an extraordinary transformation from the days when people hardly dared mention the topic for fear of being accused of racism.

The third event - if such it be - was the farcical episode when the government’s count of the new jobs taken by foreigners changed three times in a day, ending up roughly double where it began. The outcome was another blow to confidence in the government’s ability to manage immigration.

The genie is now well and truly out of the bottle. Public opinion is extremely strong - 80% disbelieve the government’s honesty and competence; 75% want to see an annual limit; two-thirds fear that our culture is under threat. Only one in three believe that immigration brings economic benefit to Britain.

The immigration lobby claims there is little that the government can do. It is all down to some mysterious force called “globalisation”. They are wrong. In fact, immigration to the UK took off in 1997. The prime cause was a series of policy errors by the present government. First, it abolished such border controls as it inherited. Then it trebled the number of work permits to 150,000 a year, plus dependants. Finally, it hopelessly miscalculated the inflow of east Europeans.

These policies can and should be reversed. The government could use its much-vaunted “points-based system” to throttle back sharply on work permits. The Australians, who have such a system, set a ceiling on immigration. So should we.

A ceiling would not apply to European Union citizens, but that is not a long-term problem. Before the recent enlargement, migration to and from the EU was roughly in balance and arrivals from the new east European members are now fairly steady at about 200,000 a year. The net inflow will decline as other EU members open their labour markets. Holland and half a dozen others have done so. The key countries, Austria, Germany and France, have kept their markets closed but restrictions have to be lifted in May 2011.

Added to that, as the economic level of east European countries approaches ours, there will be much less incentive to migrate. There was a blip for a few years when Spain, Portugal and Greece joined the EU but the net level of immigration has now fallen back.

Furthermore, those here will be more likely to go home. Many intend to spend several years here, save some money and then return to their families. As they do so, their numbers will counterbalance those still arriving so net immigration will fall.

Meanwhile, the pool of young people in eastern Europe will grow only slowly. The population of 18-year-olds in the two most populous countries, Poland and Romania, is projected to fall by about 30% in the next 10 years.

What this adds up to is that, over a decade or so, net east European immigration to the UK is likely to decline substantially. The real long-term problem is immigration from outside the EU. Here populations are growing rapidly and huge numbers of young people are without work and prospects. It is vital that our immigration system should be a barrier to such people.

Failure to act now will mean that our society will be changed beyond recognition - and especially our cities. London is one-third immigrant and half of all babies born there have a foreign parent. Other large cities will follow. According to one academic study, the ethnic community in Britain will grow from 9% to 29% by mid-century.

There is every reason for concern. The Commission for Racial Equality’s final report spoke frankly about growing segregation and of our society “fracturing”, with bonds of solidarity across different groups weakening, and tensions between people increasing. These are serious warnings. The CRE was in denial about the role of mass immigration in all this but the rest of us can see it clearly.

We can now at last speak about the elephant in the room. But when will our political leaders respond to the deep anxieties that so many of us feel? And will they get down to some serious action before it is, indeed, too late?

Sir Andrew Green is the chairman of Migrationwatch UK

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