Ministers today faced calls for an inquiry into claims that their open-door immigration policy was designed to make Britain more multicultural and allow Labour to portray the Tories as racists.
A former Labour adviser alleged that the Government opened up Britain's borders in part to try to humiliate Right-wing opponents of immigration.
The Conservatives said that if true, the claim demonstrated 'disgracefully irresponsible' decision-making and called for an investigation.
A busy shopping street
Estimates say the population of the UK could reach 70million in 20 years
Former Labour minister Frank Field said: 'I am speechless at the idea that people thought they could socially engineer a nation on this basis.'
The Daily Mail reported on Saturday the controversial claims by Andrew Neather, who worked for Tony Blair and Jack Straw.
He said Labour's relaxation of immigration controls in 2000 was a deliberate attempt to engineer a 'truly multicultural' country and plug gaps in the jobs market.
He said the 'major shift' in immigration policy was inspired by a 2001 policy paper from the Performance and Innovation Unit, a Downing Street think-tank based in the Cabinet Office.
Civil servant Jonathan Portes, who wrote the immigration report, was a speechwriter for Gordon Brown and is now an aide to Cabinet Secretary Sir Gus O'Donnell.
The report painted a rosy picture of mass immigration, stating: 'There is little evidence that native workers are harmed by migration. The broader fiscal impact is likely to be positive because a greater proportion of migrants are of working age and migrants have higher average wages than natives.'
It added: 'Most British regard immigration as having a positive effect on British culture.'
Mr Neather said the published version of the report focused on the labour market case for immigration. But he added: 'Earlier drafts I saw also included a driving political purpose: that mass immigration was the way that the Government was going to make the UK truly multicultural.'
Home Office Minister Barbara Roche, who pioneered the open-door policy, was also said to have wanted to restore her reputation on the Left of the Labour Party. She had come under fire for attacking the use of children for begging by asylum seekers as 'vile'.
Enlarge Graphic: Left-wing to the core
Ministers were reluctant to discuss the move publicly for fear that it would alienate Labour's core working-class vote, Mr Neather said. But they hoped it would allow them to paint the Conservatives as xenophobic and out of touch.
'I remember coming away from some discussions with the clear sense that the policy was intended - even if this wasn't its main purpose - to rub the Right's nose in diversity and render their arguments out of date,' Mr Neather added.
Labour strategists went on to attack Tory leaders William Hague, Iain Duncan Smith and Michael Howard as out of touch when they raised questions about immigration policy.
Mr Hague was accused of 'playing the race card' in 2001 when he said Mr Blair was turning Britain into a 'foreign land'.
Mr Howard was called a 'racist' in 2004 after he went to the BNP stronghold of Burnley to denounce Labour's stance on asylum seekers.
Mr Neather defended the open-door policy, saying mass immigration had 'enriched' Britain.
But Tory immigration spokesman Damian Green said: 'If this is true, then it would be a disgracefully irresponsible way for a government to run their immigration policy.
'To organise it on the basis of what might embarrass the Opposition would be shameful. I would urge the Home Affairs Select Committee to look at this whole episode.
'And ministers must now be honest with the British people. Do they still believe, as they did five years ago, that uncontrolled immigration is good for the country?
'If they don't, will they apologise for the mess they have made of the immigration system in the meantime?'
Jack Straw last night dismissed Mr Neather's claims as untrue.
A spokesman for the Justice Secretary said: 'This is complete rubbish and the proof of that is the fact that Jack Straw introduced and was implementing the Immigration and Asylum Act at just this time, which tightened up controls and for which he was roundly condemned by all liberals.'
However, Labour's former welfare minister Mr Field, co-chairman of the cross-party Group for Balanced Migration, said a 'beam of truth' had been shone on the immigration issue.
'It is so dangerous that I cannot believe anybody even contemplated this course of action,' he said.
'I can't believe anybody could have been this stupid. All along anyone who raised questions was told they must be racist.
'Ministers used studies like the one saying only 13,000 people would come from the EU accession countries to say we had all got our figures wrong.
'Even now people are peddling the idea that it's all over-exaggerated. The truth is that, without any changes, we are headed for a population of 70million within 20 years.'
A Home Office spokesman said: 'Our new flexible points-based system gives us greater control of those coming to work or study from outside Europe, ensuring that only those that Britain needs can come.
'Britain's borders are stronger than ever before and we are rolling out ID cards for foreign nationals.
'We have introduced civil penalties for those employing illegal workers and from the end of next year our electronic border system will monitor 95 per cent of journeys in and out of the UK.
'The British people can be confident that immigration is under control.'
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