The Labour leader used a speech in Wales to issue his frankest analysis yet of the party's failings, saying it had gone from "six people making decisions in a smoke-filled committee room in the 1980s to six people making the decisions from a sofa in Whitehall".
"Old Labour forgot about the public. New Labour forgot about the party. And, by the time we left office, we had lost touch with both," he declared.
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Hide AdHis comments came as Brown made a rare appearance to campaign in Inverclyde yesterday ahead of Thursday's by-election, with Labour hoping to prevent the SNP from making further gains after their landslide Holyrood victory last month.
The by-election was triggered by the death of Labour MP David Cairns last month, who won the seat with a 14,000 majority last year.
Labour campaigners claimed they were confident that the party can hold off an SNP surge, and retain the seat. Yesterday, the party launched an attack on David Cameron who has not yet made an appearance in the by-election campaign, claiming he was "snubbing" it.
But SNP activists in the area say they believe the result will be "very close", amid anecdotal evidence that the party may, as in last month's Holyrood election, be "leant" backing from Lib Dem, Tory and Labour supporters. In last month's Holyrood vote, held on similar boundaries, Labour won by only 500 votes.
The SNP claimed yesterday to have the momentum, releasing by-election results from councils since May which show a swing to the Nationalists, with away from Labour, the Tories and the Lib Dems.
Defeat for Labour would be a blow to Miliband as he seeks to counter criticism that his leadership is drifting.