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Labour is monitoring how many of its voters could switch to UKIP – with a detailed “Ribena test”.

In a sign of the growing threat of Nigel Farage’s party, “risk assessments” have been carried out on individuals’ potential of turning UKIP purple at the General Election on May 7.

After the top secret tests at Labour HQ , briefings were drawn up for 50 MPs whose seats are deemed most at risk.

An insider said Ed Miliband’s party was “wide awake” to the recent UKIP surge and added: “This is the most sophisticated election tool we’ve developed.

“We can compile risk assessments on a constituency by constituency basis – even taking it down to street by street.”

Labour is battling a protest vote from people angry at immigration levels while the party was last in power.

Strategists admit the party could be vulnerable across the North and Midlands.

(Image: BBC)

The three key threat areas highlighted by the Ribena test are Great Grimsby, Lincs, Rother Valley, South Yorks, and Dudley North, West Mids.

Support is even growing for UKIP in Mr Miliband’s constituency in Doncaster, where it held its autumn party conference.

The test was designed after Labour’s defeat to Ukip in last May’s European elections. Experts used data from firms and Labour’s own analysis of why some find Ukip appealing.

Now campaigners will target specific streets where they fear voters may consider switching.

As well as outlining Labour’s new tougher immigration stance, they will also highlight Mr Farage’s Tory past.

A Labour source said: “Farage is more Tory than the Tories. It is down to us to remind people that he is a Thatcherite who wants to privatise the NHS.”

UKIP insiders, however, say they are unlikely to win more than one seat from Labour.

Labour seats most at risk

Great Grimsby: Ribena Rating 10

Struggling East Coast towns are where UKIP has found the most success since 2010, winning more than 40% of the vote here at last year’s European elections and trouncing Labour.

Veteran Labour MP Austin Mitchell had a tiny majority and is stepping down. Youthful successor Melanie Onn faces a fierce battle to hold on to the seat.

Rother Valley: Ribena Rating 9

Rother Valley locals are equally angry with Labour as those in neighbouring Rotherham over the area’s child abuse scandal.

But unlike in Rotherham, there is no new-broom MP removed from past failures. Rother Valley also has fewer ethnic minority voters.

Dudley North: Ribena Rating 8

With a large ethnic minority, this Midlands seat does not seem typical UKIP territory.

But Nigel Farage’s party thrashed Labour here in the Euro elections last May.

MP Ian Austin was just three points ahead of his rival in a poll last December.

Plymouth Mount View: Ribena Rating 7

This seat offers UKIP’s best chance of taking a Labour seat in the south of England.

The party is already strong in the South West and Plymouth Mount View is the sort of coastal, working-class area where UKIP does well. One poll last year put UKIP marginally ahead against Labour MP Alison Seabeck, who was elected in 2005.

Rotherham: Ribena Rating 7

Labour has held this South Yorkshire town for decades but face deep anger over the council’s failure to deal with child abuse . UKIP already forms the main opposition at the town hall.

But MP Sarah Champion was elected only in 2012 and is untarnished by the past.

Heywood & Middleton: Ribena Rating 6

UKIP came within a whisker of taking this North West seat at a by-election last year, missing out by just 617 votes. Received wisdom is that vote was a high-water mark for UKIP, and that Labour will now hold on comfortably.

Ashfield: Ribena Rating 5

Labour has a tiny majority in this former Notts mining constituency, and there was a big anti-Europe vote last year.

But MP Gloria De Piero is a strong campaigner.