The huge increases in migrants over the last decade were partly due to a politically motivated attempt by ministers to radically change the country and “rub the Right’s nose in diversity”, according to Andrew Neather, a former adviser to Tony Blair, Jack Straw and David Blunkett.
He said Labour’s relaxation of controls was a deliberate plan to “open up the UK to mass migration” but that ministers were nervous and reluctant to discuss such a move publicly for fear it would alienate its “core working class vote”.
As a result, the public argument for immigration concentrated instead on the economic benefits and need for more migrants.
Critics said the revelations showed a “conspiracy” within Government to impose mass immigration for “cynical” political reasons.
Mr Neather was a speech writer who worked in Downing Street for Tony Blair and in the Home Office for Jack Straw and David Blunkett, in the early 2000s.
Writing in the Evening Standard, he revealed the “major shift” in immigration policy came after the publication of a policy paper from the Performance and Innovation Unit, a Downing Street think tank based in the Cabinet Office, in 2001.
He wrote a major speech for Barbara Roche, the then immigration minister, in 2000, which was largely based on drafts of the report.
He said the final published version of the report promoted the labour market case for immigration but unpublished versions contained additional reasons, he said.
He wrote: “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.
“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was intended – even if this wasn’t its main purpose – to rub the Right’s nose in diversity and render their arguments out of date.”
The “deliberate policy”, from late 2000 until “at least February last year”, when the new points based system was introduced, was to open up the UK to mass migration, he said.
Some 2.3 million migrants have been added to the population since then, according to Whitehall estimates quietly slipped out last month.
On Question Time on Thursday, Mr Straw was repeatedly quizzed about whether Labour’s immigration policies had left the door open for the BNP.
In his column, Mr Neather said that as well as bringing in hundreds of thousands more migrants to plug labour market gaps, there was also a “driving political purpose” behind immigration policy.
He defended the policy, saying mass immigration has “enriched” Britain, and made London a more attractive and cosmopolitan place.
But he acknowledged that “nervous” ministers made no mention of the policy at the time for fear of alienating Labour voters.
“Part by accident, part by design, the Government had created its longed-for immigration boom.
“But ministers wouldn’t talk about it. In part they probably realised the conservatism of their core voters: while ministers might have been passionately in favour of a more diverse society, it wasn’t necessarily a debate they wanted to have in working men’s clubs in Sheffield or Sunderland.”
Sir Andrew Green, chairman of the Migrationwatch think tank, said: “Now at least the truth is out, and it’s dynamite.
“Many have long suspected that mass immigration under Labour was not just a cock up but also a conspiracy. They were right.
“This Government has admitted three million immigrants for cynical political reasons concealed by dodgy economic camouflage.”
The chairmen of the cross-party Group for Balanced Migration, MPs Frank Field and Nicholas Soames, said: “We welcome this statement by an ex-adviser, which the whole country knows to be true.
“It is the first beam of truth that has officially been shone on the immigration issue in Britain.”
A Home Office spokesman said: “Our new flexible points based system gives us greater control on those coming to work or study from outside Europe, ensuring that only those that Britain need can come.
“Britain’s borders are stronger than ever before and we are rolling out ID cards to 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.”