Labour has seized the Corby constituency from the Conservatives by a margin of almost 8,000 votes in a major boost for leader Ed Miliband.
It was the first time in 15 years that Labour has won a House of Commons seat from the Tories in a by-election and victorious candidate Andy Sawford suggested it had set the party on track for a successful general election in 2015, declaring: "The road to Downing Street runs through Corby."
The by-election in the Northamptonshire seat was forced by the resignation of Conservative MP Louise Mensch to spend more time with her family in the United States.
On Thursday night she took to Twitter to accept the blame for her party's failure to retain the seat, which was Labour from 1997 until 2010.
But Mr Sawford said the result was "a damning verdict on (David) Cameron's betrayal of the British people".
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."