Lord Green of Deddington said Britain will be forced to build three cities the size of Birmingham within five years to cope with current levels of immigration

Britain will be forced to build three cities the size of Birmingham within five years to cope with current levels of immigration, experts warned last night.

Lord Green of Deddington, chairman of think-tank MigrationWatch UK, said the population could increase by a staggering 3million by 2020 – adding strain to the country’s creaking public services.

The prediction will heap pressure on Prime Minister David Cameron to take control of the borders as part of his renegotiation of Britain's membership of the EU.

Immigration now tops the list of voters' concerns in recent polls – with nearly 6 in 10 saying tackling immigration is among the most crucial issues facing the UK.

But his pledge to cut net migration to the tens of thousands has been left in tatters after the number soared to a record 330,000 in the year to March – a rise of 40 per cent in one year.

Lord Green said that if net migration – the number of people coming into the country minus those leaving – and the natural population increase continued at today’s rates, it would mean a typical rise of around 600,000 a year.

Giving evidence to the Home Affairs Select Committee, he said: ‘On current levels of net migration, we are going to have to build three cities the size of Birmingham within five years. That just can’t be done.’

Birmingham, the UK’s second largest city, has a population of just over 1million, according to the 2011 census.

Lord Green said it would be ‘very difficult’ for the Government to hit its 100,000 migration target but he opposed axing the figure, arguing that it ‘focused’ bureaucrats and public opinion.

‘Unless some action is taken to reduce the inflow from European countries, it will not be possible to bring levels of migration down to levels that people in this country will accept,’ he said.

However, Boris Johnson has warned Britain that low immigration can lead to Japanese style economic stagnation.

Despite being the world's third largest economy the country has suffered from a lack of growth and the population size is falling, the mayor said on a trade visit to Tokyo.

'Britons must think about the consequences of curbing the number of people from overseas that move to the UK,' he insisted.

It comes just a week after Home Secretary Theresa May gave a stark speech to the Conservative Party conference warning that mass immigration made it impossible to build a cohesive society - comments Mr Johnson was quick to distance himself from.

Speaking in Tokyo, the mayor said: 'They have been going through a long period of stagnation but they are hoping to pull out of it.

Put into context: Birmingham, the UK’s second largest city (pictured), has a population of just over 1million

"They have got demographic problems. One of the questions that people in Britain might think about is obviously that they have very, very low immigration and very, very low, in fact negative, population growth, they have got a shrinking population.

'That has, of course, contributed to the long period of economic stagnation they are going through.'

The latest Office for National Statistics figures, published in August, showed an increase in net migration from inside the EU to 183,000, up 53,000, over the year.

Mr Cameron wants to curb migrants' access to benefits as part of his efforts to claw back powers from Brussels, before holding an in-out referendum by the end of 2017.

Meanwhile, Labour voted against tough new immigration rules last night in a sign the divided party is out of touch with voters.

Home Secretary Theresa May’s Immigration Bill included a crackdown on illegal foreign workers, restrictions to bank accounts and driving licences and proposals to dock the wages of migrants caught working unlawfully.

It also included a controversial requirement for landlords to check the immigration status of tenants – raising fears it could lead to discrimination, a claim Mrs May dismissed.