Image caption The UK's population is projected to reach 70 million within the next 23 years

MPs must take their heads "out of the sand" on immigration, senior Conservative Nicholas Soames has said.

Mr Soames opened a Commons debate prompted by an online petition urging the government to ensure that the UK's population remained below 70 million.

More than 100,000 people signed the petition in less than a week, he said.

But Labour's Chris Bryant warned ministers that their planned curbs on immigration would be very unlikely to achieve the petitioners' aim.

The government has pledged to cut net migration - the difference between the number of people leaving the UK and those moving to the UK - down to the "tens of thousands" by 2015.

But Mr Bryant told MPs: "The fact is that that if net migration were zero in every category for the next 25 years, the population would grow to 66 million.

"And if it were tens of thousands... the population would be 70 million just after 2035."

Projected UK population 2010-35

Mr Soames had blamed the previous Labour administration for its "chaotic, ill-thought out and deeply irresponsible approach to immigration".

Under its watch, he said, the UK had witnessed "the greatest wave of immigration... in nearly 1,000 years".

The popularity of the petition, which calls for "all necessary measures" to be taken to ensure the population stays "well below" 70 million and has been signed by more than 143,000 people, had provided a "clear indicator of the very grave public concern about the scale of immigration to this country", Mr Soames added.

Although immigration was a "natural and essential part of an open economy" with some benefits, there were pragmatic causes for concern, he said.

He told MPs: "In the coming 15 years we will have to build, just for new immigrants and their families, the equivalent of eight of the largest cities outside London... together with all their associated social infrastructure, of schools, roads, hospitals, railways, and all the rest."

Mr Soames was backed by Labour MP Frank Field, who said that the government's bid to reduce net migration rates had garnered cross-party support.

The two MPs, along with eight parliamentary colleagues, tabled a Commons motion for debate, based on the online petition.

But SNP MP Pete Wishart warned that their adoption of the phrase "all necessary steps" in their "nasty little motion" sounded worryingly authoritarian.

'Terrible thoughts'

Mr Field contrasted the attitude to the UK of Olympians like Mo Farah with the terrorists responsible for the 2005 London bombings.

He asked why "we have so many people who come here and are so committed [to the UK], and yet at the same time there are some... second generation [immigrants] who harbour such terrible thoughts in their hearts about us that they actually want to take terrible action against us".

His Labour colleague Diane Abbott was prompted by this comment to intervene.

"What possible evidence does [he] have that more than a tiny fraction of a fraction of second generation migrants harbour terrible thoughts?" she asked.

Mr Field said he accepted there was "no evidence" to contradict her proposition.

But Lib Dem MP Martin Horwood said he was "concerned about the tone" of Mr Soames and Mr Field's speeches.

"Would he agree that actually immigrants can make a very positive contribution to our economy, and to our culture, and we need to take a balanced, evidence-based approach to this whole debate, and not use language that will inflame fears amongst minority ethnic communities in this country?" he asked Mr Field.

The Labour MP responded: "There is not the case that unlimited migration of the scale that we've seen is such an economic advantage to this country as some of those proponents of open doors would wish us to believe."

Net migration by citizenship 1991 - 2011

Migration Watch, a think tank which campaigns for tighter immigration controls, is concerned at the effect the UK's rising population would have on the country's infrastructure and quality of life, and so organised the petition.

It argues that net migration needs to be limited to 40,000 to keep the UK population permanently below 70 million.

Figures published by the Office for National Statistics (ONS) show that the UK population would rise from 62.3 million in 2010 to 67.2 million by 2020 and 73.2 million by 2035 if net migration stays at about current levels.

ONS figures released last month say estimated net migration in the year to December 2011 was 216,000 - down from 252,000 the previous year.

It said the fall was not statistically significant, but ministers argue separate figures show they are on course to reach their target of reducing net migration to under 100,000 people a year by 2015.

Ahead of the Commons debate, Martin Ruhs, director of Oxford University's Migration Observatory, urged MPs to "move beyond rhetoric and into substance" and consider the "trade-offs associated with the deep cuts to net migration that the 70 million limit would require".