The Labour leader, Ed Miliband, drew comfort yesterday from the party's decisive victory in the Feltham and Heston by-election, calling it a "verdict on the Government's failed economic plan".
But the contest is likely to be best remembered for its dismal turnout, with less than 29 per cent of electors in the west London constituency bothering to vote.
Labour's Seema Malhotra won with a majority of 6,203 over the Conservatives, while the Liberal Democrats saw off a challenge from the UK Independence Party to cling on to third place.
Ms Malhotra, a management consultant, achieved a swing of 8.6 per cent compared with the general election result, although the number of votes cast for Labour fell because of the lowest turnout in a by-election for 11 years.
Mr Miliband, who is facing growing muttering over his leadership, told supporters: "This is not just a swing from the Tories to Labour. It is a swing away from Tory ideas to Labour ideas. Labour is starting to win back trust but we know there is more to do, more arguments we need to win, more people we have to convince, more people we have to win back."
The Tories, whose share of the vote dropped by six per cent, conceded their performance had been disappointing, but said Labour needed to win by a much larger margin to prove they were on course for election victory.
There was relief for the Liberal Democrats, who avoided the humiliation of being forced into fourth place by Ukip by 88 votes. Roger Crouch, the Liberal Democrat candidate, said: "We have shown we are still resilient and still relevant." But Ukip, whose support may have been depressed by David Cameron's stance in last week's European summit, said the collapse in the Liberal Democrat vote proved the party was a "busted flush".
The by-election was caused by the death from cancer last month of Alan Keen, who had represented the seat since 1992. He retained it last year with a majority of 4,658.
The turnout of 28.8 per cent – the lowest since the West Bromwich West by-election in November 2000 – was described by the Conservative chairman, Baroness Warsi, as "very worrying".