Immigration is the issue many in Labour fear most

Ukip is threatening certain Labour seats at the next election

The influx of hundreds of thousands of eastern European workers has significantly altered the composition of some parts of Britain (Getty Images)

For some Labour MPs, the biggest single mistake of Tony Blair’s premiership wasn’t going to war in Iraq or allowing the national deficit to escalate.

Instead they look back with a shiver to the decision to open Britain’s borders in 2004 following the admission of Poland and seven other former Iron Curtain countries to the European Union.

The influx of hundreds of thousands of eastern European workers helped to boost the economy, but also significantly altered the composition of some parts of Britain.

Labour received few bouquets for the former, but plenty of brickbats for the latter – symbolised by Gordon Brown’s confrontation with “that bigoted woman” Gillian Duffy during the last election.

Throughout the Blair and Brown years, the party leadership had been reluctant to speak about immigration. That reticence has now been replaced by apologies for the past and a toughening in Labour’s language, culminating in Ed Miliband’s call for “stronger controls on people coming here”.

The power of anti-migrant prejudice was illustrated 50 years ago when the Tories infamously captured the West Midlands seat of Smethwick from Labour under the slogan: “If you want a nigger for a neighbour, vote Labour.”

Shortly afterwards, dockers from London’s East End – a spiritual home for Labour – marched in support of Enoch Powell’s apocalyptic “rivers of blood” vision of the impact of immigration.

More recently, it was no coincidence that the British National Party made spectacular, if short-lived, inroads in former Labour areas such as Barking, Burnley and Stoke.

Now the UK Independence Party is exploiting the same disillusionment and threatening to take several Labour seats in next May’s general election.