The reason Labour's lead is shrinking? It's losing battle to keep hold of voters gained from Lib Dems after formation of Coalition

The Independent 'poll of polls' suggests 'lost' supporters are mainly switching to Green Party or Ukip

Labour is losing the backing of the Liberal Democrat supporters who flocked to it after Nick Clegg formed the Coalition with the Conservatives, according to The Independent's latest "poll of polls".

At the start of this year, about 39 per cent of people who voted Lib Dem at the 2010 election said they would now back Labour. But the figure has now dropped to about 29 per cent.

The trend might help to explain why Labour's lead over the Tories has shrunk from 13 points in February to five points, and why it has not grown significantly since Mr Miliband announced his popular pledge to freeze energy prices for 20 months at the party's conference in September.  That has set the political agenda since and boosted Mr Miliband's previously weak personal ratings.

John Curtice, professor of politics at Strathclyde University, who compiled the "poll of polls", said Labour's loss of support among Lib Dem 2010 voters is big enough to cost it almost 2.5 per cent of the total vote - which could prove crucial in a tight general election. He said the polls suggest Labour's "lost" Lib Dem voters have switched mainly to the Green Party or the UK Independence Party. Some eight per cent of former Lib Dem supporters would now vote Green, up from five per cent in February. The Greens are now running at three to four per cent in the polls -- by historical standards, a relatively healthy showing for the party. Ukip now has the backing of eight per cent of 2010 Lib Dem voters, up from four per cent in January.

"Labour cannot assume that it is the only possible beneficiary of the Lib Dems' misfortunes," said Professor Curtice. "It is not just David Cameron who has to worry about the challenge from parties beyond the Westminster bubble. It could yet scupper Ed Miliband's chances too." The proportion of Labour 2010 voters switching to Ukip has risen from two to five per cent since the start of the year.

The "poll of polls," based on a weighted average of surveys by ComRes, ICM, Ipsos MORI and YouGov, still points to a Labour majority of 50. But some Labour MPs fear that could melt away in the heat of an election battle.

Professor Curtice said: "The decline in Labour support during the course of this year must now be regarded as one of the key long-term developments in party fortunes, on a par with the collapse of the Lib Dem vote in the autumn of 2010, the decline in Conservatives' fortunes in the spring of 2012 and the rise in Ukip support in the twelve months thereafter."

A Labour source denied that the party was losing the backing of former Lib Dem voters. "Our support is much more solid since the party conference and Ed Miliband's personal ratings have been transformed," he said. Labour strategists hope the heated political debate on the "cost of living crisis" will enable the party to widen its lead over the Tories.

Lib Dem officials said the party hoped to win back some of the 2010 supporters who defected to Labour when Mr Clegg entered the Coalition. "We normally improve our ratings during a campaign as more people take a look at us," one said. The Lib Dems will target "soft Labour" supporters in Con-Lib Dem marginals. They believe some Labour sympathisers will vote tactically to keep the Tories out, as they did at the Eastleigh by-election in February, when the Labour vote collapsed and Mr Clegg's party held the seat.

Meanwhile, a Labour Party motion calling on ministers to introduce a 20-month price freeze on energy bills was defeated by 295 votes to 237, a Government majority 58.

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