Conservative leader David Cameron has hailed his party's victory in Norwich North as "historic" and branded Labour's campaign as "despicable".
Mr Cameron said by-election winner Chloe Smith, 27, was a credit to the party and the kind of MP he wanted.
She became the country's youngest MP by overturning Labour's 5,000-plus majority to win by 7,348 votes in the first such test of public opinion since the Westminster expenses scandal.
Mr Cameron, who travelled to Norwich to celebrate the by-election win with party members, said: "I think we have seen an absolutely historic result here in Norwich North."
The Tory leader also criticised Labour for telling "untruth after untruth" about Conservative policies during the campaign and criticised Prime Minister Gordon Brown for not visiting the constituency.
"I have seen a Labour campaign in this by-election that I would describe, and I choose my words carefully, as utterly despicable," he said. "If you look at what they said about us it was untruth after untruth.
"I say to this Prime Minister who talks about courage and who talks about a moral compass: where was the courage in not even coming to this by-election and where is the moral compass in allowing a campaign of lies and half truths about your opponents?"
Mr Brown said Labour's defeat was a "disappointing result". The Prime Minister said voters were torn between their "anger and dismay" at the Westminster expenses scandal and the popularity of the previous Labour MP.
The poll was forced by the resignation of Labour MP Ian Gibson, who claimed almost £80,000 in second home expenses on a London flat which he later sold at a knock-down price to his daughter.
He quit last month after Labour's expenses "star chamber" barred him from standing for the party, sparking fury among colleagues and activists who said he was harshly treated by the "kangaroo court".
