Labour swept to a comfortable by-election victory as it captured the previously marginal seat of Corby from the Tories with a majority of almost 8,000.
Ed Miliband, the party’s leader, told crowds in the Northamptonshire constituency that the result proved Middle England was turning its back on David Cameron and the Conservatives.
Labour had poured massive resources into winning a seat which is regarded as a traditional political bellwether – and the scale of its win will be a major boost to party morale.
The Tories’ problems were exacerbated by the circumstances of the by-election, which was caused by Louise Mensch’s surprise decision to resign to join her rock manager husband in New York.
Labour’s Andy Sawford overturned a Tory majority of 1,951 at the last general election with a swing of more than 12.6 per cent.
He collected 17,267 votes, with Tory candidate Christine Emmett, who was embarrassed when it emerged that her campaign manager had offered support for an anti-wind power rival, polling 9,476 votes.
The UK Independence Party achieved its best-ever by-election performance, receiving 5,108 votes and pushing the Liberal Democrats into a distant fourth. Nick Clegg’s party suffered the embarrassment of losing its deposit, despite requesting two recounts.
The Conservatives sought to play down the result, insisting they were badly affected by the “Mensch factor”. Mr Cameron said: “It’s a classic mid-term result and obviously made difficult by the fact that the Conservative MP left the seat in question.”
Before the result was declared, Ms Mensch also attempted to shoulder the blame. She said: “Though I had to resign to unite our family, there was large & entirely understandable local anger. I wish I had been able to see out my term. Christine and her team did an incredible job in the most difficult possible circumstances. I know she will one day make a wonderful MP.”
But Mr Miliband said: “This constituency is at the heart of our country and this constituency has sent a very clear message today. It has sent a message that it is putting its trust in a One Nation Labour Party and middle England is turning away from David Cameron and the Conservatives because middle England feels let down by David Cameron and the Conservatives.”
Labour also scored comfortable victories in by-elections in two other safe seats.
The party held Manchester Central with a majority of 9,936 votes and Cardiff South & Penarth by 5,334 votes. Both contests were triggered by the decision of sitting Labour MPs to resign to stand in the police and crime commissioner elections.
Lucy Powell picked up 11,507 votes in the Manchester contest, almost 70 per cent of the votes cast.
The turnout of 18.2 per cent was reportedly the lowest in a parliamentary by-election since the Second World War.
In Cardiff South, Stephen Doughty with 9,193 votes was well ahead of his Tory rival who won 3,859 votes.