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 deliberate plan to “open up the UK to mass migration”.

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 said.

Ministers hoped to radically change the country and by doing so “rub the Right’s nose in diversity”, added Mr Neather, but senior Labour figures remained reluctant to discuss the policy in case it alienated the party’s “core working class vote”.

Critics said the revelations showed a “conspiracy” within Government to impose mass immigration for “cynical” political reasons.

During his Thursday night appearance 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.

Read More: Mass immigration “for political gain”, says adviser