Brown, whose critics paint him as socially unskilled and bullying, was caught on tape on Wednesday complaining about a woman in her 60s who had challenged his 13-year-old Labour government's approach to immigration.
"She's just sort of a bigoted woman," Brown was unwittingly recorded as saying when he got into his car after meeting Gillian Duffy, a voter in the northern town of Rochdale.
Brown later apologized in person, but his remorse may not convince many voters before the May 6 ballot, heaping pressure on him to claw back lost ground in Thursday's televised leaders' debate, the last of three before the election.
The U.S.-style debates have dominated campaigning, and the final debate at 1930 GMT will focus on the economy, the most important issue of the election as contenders tout plans to cut Britain's record budget deficit.
Center-left Labour has trailed the center-right Conservative Party in opinion polls, and even the much smaller centrist Liberal Democrats, or Lib Dems, according to some surveys.
British bookmakers sharply lengthened their odds against Labour securing a majority in the election after Brown's gaffe, although some voters and pundits were more sympathetic, playing down his outburst as a mistake anyone could make under pressure.
Lib Dem leader Nick Clegg's popularity has surged after the first two debates, alarming Labour and the Conservatives, who have warned that a vote for the Lib Dems would make an inconclusive election and so called "hung parliament" more likely.
Opinion polls have consistently pointed to such a result in recent weeks, an outcome unseen in British politics since 1974.
Led by David Cameron, the Conservatives say that would unnerve markets and harm Britain's economy.
The media will scrutinize the TV debate for any clues to the extent or timing of budget cuts, expected after the election to rein in a budget deficit running at more than 11 percent of GDP.
Politicians have been wary of stoking voters' wrath by being too candid about what are expected to be swinging cuts.
The Conservatives want to present an emergency budget soon after the vote to cut spending, a move they say will safeguard Britain's triple A credit rating.
Both the Conservatives and the Liberal Democrats warn that Britain risks a Greek-style economic meltdown if the budget deficit is not tackled.
Labour points to signs of economic growth, which they say will be put at risk if deep budget cuts are made too soon.