Lord Mandelson said the Norwich North by-election was "peculiar"
Labour's defeat in the Norwich North by-election will not reopen questions about Gordon Brown's leadership of the party, Lord Mandelson has said.
Several Labour MPs have accused Mr Brown of being directly responsible for the loss of the seat to the Tories.
The business secretary said he was "sorry" about the outcome but the poll took place in "unique" circumstances.
While he acknowledged some MPs would continue to attack Mr Brown, he said they were very much in a minority.
Ex-home secretary Charles Clarke blamed the Norwich result, which saw the Tories take the seat with a 16.5% swing, on Mr Brown's "incompetent" treatment of outgoing MP Dr Ian Gibson.
Among others to pin the blame on the prime minister was Barry Sheerman, who said the result was a "self-inflicted wound" and warned Mr Brown had until the end of the summer to reconnect with voters.
But commenting on the result, Lord Mandelson - appointed first secretary of state in June's reshuffle - said it would not affect Mr Brown's position.
"It was one of the most peculiar by-elections with some of the most unique elements I have ever seen," he told the BBC's World At One programme.
Labour voters had felt "torn" over what to do due to their anger over the general conduct of MPs and their loyalty to Dr Gibson as a local MP, he said.
"I am sorry that so many Labour voters chose to do this by staying at home, with the result you have got."
Despite the Tories' conclusive victory, Lord Mandelson said none of the main parties had much to "cheer" about as smaller parties were the main beneficiaries of popular anger over expenses.
'Handful of critics'
The outcome had changed nothing in relation to Mr Brown's leadership, he continued.
Although he did not expect the "handful" of MPs who were criticising the prime minister to back off, Lord Mandelson said they did not represent mainstream views.
"I don't have a question about Gordon Brown's leadership nor do the overwhelming bulk of members of the Labour Party and those who represent it in Parliament," he added.
Mr Brown has acknowledged the result was affected by the anger of many local Labour supporters at the party executive's treatment of their former MP.
Dr Gibson had resigned after being told he could not stand for election again by an internal Labour panel following questions over his expenses.