David Cameron’s smooth ride to the Conservatives’ first by-election victory over Labour in 30 years hit a bump last night when his party admitted sending personal information on 8,000 voters to a radio station by mistake.

A Tory spokesman declined to say what information was disclosed, but it is thought to include addresses, telephone numbers and coded information on likely voting intentions.

Ed Miliband, the Cabinet Office minister, wrote to Mr Cameron urging him to apologise for the blunder.

A Tory spokesman said that the party was investigating the error. “The e-mail, which was based on information from the electoral register, was sent in error to a journalist. Within 2½ hours the recipient was informed. Both he and the local newspaper he sent it on to have now given undertakings that the information on the e-mail has been destroyed.”

Labour was clinging to the slender hope that the gaffe may yet deny Mr Cameron victory. Sources within the party had predicted that the Tories would win by a margin of 7,078 votes – Labour’s present majority – while Eric Pickles, the Tory campaign chief, said that a win for his party would bury for good the notion that “the Cameron brand” had little appeal in the North.

The by-election was triggered by the death of Gwyneth Dunwoody last month. Labour sought to defend the seat by rushing forward the election date and selecting Mrs Dunwoody’s daughter, Tamsin, as its candidate.

But the row over the abolition of the 10p tax cut, as well as rising fuel and food costs, have been a grim background to the campaign and efforts to caricature the Tory candidate, Edward Timpson, and David Cameron as “toffs” have back-fired. Internal recriminations surfaced at the weekend as both Mr Miliband and Harriet Harman, Labour’s Deputy Leader, distanced themselves from the campaign.

Particularly worrying for Mr Brown is evidence that large numbers of younger Labour voters – aged between 25 and 35 – are planning to switch their votes to Mr Cameron.

Mr Brown discussed the likely defeat with the party’s parliamentary committee yesterday. The group of Labour’s most senior backbenchers told the Prime Minister to hold his nerve in the face of the loss of Crewe & Nantwich. They said that there should be no outbreak of infighting and that he had been right to order the tax U-turn and to improve rights for agency workers. Privately, however, even Mr Brown’s allies concede that he is failing to explain clearly to voters how he planned to help them with the rising cost of living.

Steven McCabe, the MP chosen by Mr Brown to run the campaign, told The Times that it had been “very difficult to buck the national trend”. He defended having dressed activists in top hats and tails, insisting that far from signalling a class war, the stunt had been intended as “an instance of humour”.

“I think it’s been very hard to buck the national trend. We’ve not been helped by an uncritical media that has accepted Tory propaganda, which has also been accepted by armchair strategists. We had three weeks to draw a quite stark dividing line coming from a very, very low base after the local election results.”

Tamsin Dunwoody was “tailor-made to be an MP”, he said, whereas Mr Timpson was a “nodding dog” and it had been right to draw attention to the difference.

In a dig at those who had anonymously criticised the use of class, he said: “If anyone wanted to know what was really going on, they should have joined those who came to lend a hand.” He said reports that Mr Brown had authorised class-war attacks as a dry run for the general election were “fictitious drivel”.

Speaking before news emerged of his party’s blunder, Mr Pickles said that the by-election had been “one of the most bizarre” he had known. Mr Pickles, who is credited for master-minding the Tories’ local elections triumph, was keen to praise Stephen Gilbert, a backroom official in charge of the Crewe campaign logistics.

“Lessons one, two and three are that there are no magic bullets. There has been no single leaflet or carefully crafted message that has made the difference. It’s been an old-fashioned campaign: just getting out on the knocker.”

He said that the Tories had managed to squeeze the Liberal Democrat and Labour votes alike, tailoring different messages for each. Lib Dems had been “love-bombed”, he said.

“They know that we share their aims on issues like social justice and the environment.” For Labour voters, the appeal had been even more straightforward: “To send the Prime Minister a message”.

The Tories had profited from Labour’s inability to connect to voters. “Labour just quotes statistics at people. We have a Prime Minister who knows the price of a barrel of oil but can’t tell you what a litre of petrol costs,” he said.

Mr Pickles said that he had detected a more fundamental shift in attitudes, that it was impossible to win the seat just by exploiting the greater willingness of Tories to vote. “This is 165 on our list of target seats – we need Labour switchers.”

A senior Labour source said: “People are feeling the pinch. There is a major economic downturn and we haven’t so far been able to explain why it’s happened and what our remedy is. Voter anger is being articulated around the 10p tax rate and you don’t turn around something like that in a couple of weeks.”

Crewe & Nantwich

ComRes Poll (May 20):

Conservative 48%

Labour 35%

Liberal Democrats 12%

2005 general election result:

Conservative 32.6% - 14,162

Labour 48.8% - 21,240

Liberal Democrat 18.6% - 8,083

— Crewe is home to an engine works and the Bombardier rolling stock company as well being a major railway junction

— Gwyneth Dunwoody held Crewe for Labour from 1974, and Crewe & Nantwich from its creation in 1983 until she died in April

— It was rated by the Conservatives as the 165th most winnable seat in their target constituencies

Source: Times archives; ComRes Poll

Times Recommends

• Everyone will get vaccine against swine flu
• Artificial sperm and the race to build a baby
• BBC jets out stars to tout for prizes