• Conservative victor has 7,000-plus majority
• Tory leader says Brown ‘should learn lesson’

David Cameron accused Gordon Brown of running an “utterly despicable” campaign in Norwich North today as he celebrated a byelection victory that saw the Conservatives winning what was a safe Labour seat with a majority of more than 7,000. The Tory leader claimed that Labour had told “untruth after untruth” about opposition spending plans in the contest, which was triggered by the resignation of Ian Gibson after he was banned by his party from standing at the general election because of the way he used parliamentary expenses.

Chloe Smith, who at 27 becomes the youngest MP in Britain, took the seat with a swing from Labour to the Tories of 16.5%. Gibson had a majority of more than 5,000 at the last election and Norwich North has been Labour for 45 of the last 60 years.

Smith took nearly 40% of the vote, although when the result was declared at lunchtime on Friday it was clear that she had picked up fewer votes than the Conservative candidate did in 2005. Labour’s Chris Ostrowski, who was struck down with swine flu in the final 72 hours of the campaign – his wife Katie delivered his speech at the count – got just 18% of the vote, although he managed to see off a challenge from the Liberal Democrats. They had hoped for second place but got 14% of the vote and third place.

The campaign started after the Conservatives declared that they might have to cut public spending in most government departments by 10% after the general election, and Labour attacked Smith aggressively on this issue. One Labour leaflet suggested that the Tories could close up to 10% of schools in the country, and another said the Tories were “threatening to do away with free TV licences and bus passes for the elderly”.

This Cameron, on a celebration visit to Norwich, condemned Labour’s tactics in the strongest terms.

“I have seen a Labour campaign in this byelection that I would describe – and I choose my words carefully – as utterly despicable. If you look at what they said about us it was untruth after untruth,” he said. “Labour should learn a lesson … in this campaign where less than one in five people in a Labour-held constituency came out to vote for the Labour party, that this country has had enough of Gordon Brown’s dividing lines, has had enough of Gordon Brown’s misleading claims about his opponents, has had enough of Gordon Brown’s claims about Tory cuts and Labour investment and all the rest of that rubbish.”

Tory strategists believe that Brown was using the byelection to road-test a “Tory cuts” campaign and that the result shows that this approach does not work.

The byelection was also the first to be held since the controversy about MPs’ expenses erupted. The Tories believe their victory proves that main-party candidates such as Smith, who made transparency a key feature of her campaign, can still see off the threat from independents and minor parties in an era of public scepticism about politicians.

Brown said the result was “disappointing” for Labour but that local factors were to blame. “The voters were clearly torn between their anger and dismay at what has been happening over MPs’ expenses, something that we are trying to clean up, and at the same time the support for the former MP, the Labour MP Ian Gibson who was very popular,” he said. Brown also pointed out that all the main parties attracted fewer votes than they did in the seat in 2005.

The defeat seems unlikely to reopen the debate about Labour’s leadership, at least in public and in the short-term. David Miliband, the foreign secretary, said today: “Everybody understands that the byelection reflects some unique circumstances. It is not evidence for the need of a change in the Labour leadership.”

The Liberal Democrat leader, Nick Clegg, said Brown’s “ham-fisted treatment of a popular MP” had resulted in “disaster for Labour”.

According to a Press Association analysis, Cameron would be swept into 10 Downing Street with a Commons majority of 218 if the result was repeated across the country at the next general election.