Huge increases in immigration over the past decade were a deliberate attempt to engineer a more multicultural Britain, a former Government adviser said yesterday.Andrew Neather, a speechwriter who worked in Downing Street for Tony Blair and in the Home Office for Jack Straw and David Blunkett, said Labour`s relaxation of controls was a plan to `open up the UK to mass migration`.As well as bringing in hundreds of thousands to plug labour market gaps, there was also a `driving political purpose` behind immigration policy, he claimed.Ministers hoped to change the country radically and `rub the Right`s nose in diversity`. But Mr Neather said senior Labour figures were reluctant to discuss the policy, fearing it would alienate its `core working-class vote`. On Question Time, Mr Straw was repeatedly quizzed about whether Labour`s immigration policies had left the door open for the BNP.Writing in the Evening Standard, Mr Neather 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based in the Cabinet Office.The published version promoted the labour-market case for immigration but Mr Neather said unpublished versions contained additional reasons.`Earlier drafts I saw also included a driving political purpose: that mass immigration was the way that the Government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was intended - even if this wasn`t its main purpose - to rub the Right`s nose in diversity and render their arguments out of date.` The `deliberate policy`, from late 2000 until `at least February last year`, when the new points-based system was introduced, was to open up the UK to mass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.Mr Neather defended the policy, saying mass immigration has `enriched` Britain and made London a more attractive and cosmopolitan place.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 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 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.`Who knew?