Ed Miliband has said a Labour MP who tweeted a picture of a house with three England flags and a white van parked outside was "disrespectful".
Emily Thornberry quit Labour's shadow cabinet and subsequently apologised over the picture, which was branded "snobby" by the family living there.
The Labour leader said he was "furious" about the tweet, which gave a "misleading impression".
Labour has "a challenge in relation to UKIP", he admitted.
Writing in the Daily Mirror, he added it was "a challenge I'm determined to meet".
"I am not going to let UKIP get away with the claim they will stand up for working people," he pledged on Friday.
Ms Thornberry posted the image on Thursday, while voting was taking place in the Rochester by-election. Alongside the picture, she wrote: "Image from Rochester".
Labour came third in the high-profile poll in Kent behind UKIP, which won the seat and saw its second MP elected to Westminster.
Speaking outside her London home on Friday, Ms Thornberry said she had "made a mistake" and apologised "if she had upset or insulted anybody".
But in the immediate aftermath of her resignation, she 'favourited' a number of tweets which lamented her standing down.
The resident of the house, Dan Ware, said Ms Thornberry - the MP for Islington South and Finsbury - was a "snob" while her actions were also criticised by Conservative leader David Cameron and UKIP leader Nigel Farage.
Ms Thornberry is believed to have had two conversations with Mr Miliband on Thursday after posting the tweet, and offered her resignation during the second one.
The Labour leader said that although the MP may have not intended to cause offence, her actions had "conveyed a sense of disrespect".
"I was angry because I thought her tweet gave a misleading impression, when she photographed the house in which the family lived, that somehow Labour had the wrong view of that family," he told reporters as he entered a meeting in London earlier on Friday.
"It's not the view we have of that family. Labour's never had that view of disrespect. I'm afraid her tweet conveyed a sense of disrespect."
Asked what reaction he felt when he saw such an image, Mr Miliband said "respect".
He added: "I thought there was nothing unusual or odd, as her tweet implied, about having England flags in your window.
"That's why I was so angry about it and that's why I think it's right that she resigned."
Mr Ware, a car dealer, said he would never vote for Labour in the future, adding that it did not matter who was in government.
"I've not got a clue who she is - but she's a snob," he told the Sun. "We put the flags up for the World Cup and will continue to fly them."
"I think they (Labour) need to get out of their mansions and visit the working class. Her and Ed (Miliband) should come and say sorry to me."
'Respect for voters'
BBC political editor Nick Robinson said the tweet had "given the Tory press an alternative narrative" to the party's defeat in Rochester.
He said: "It is the most extraordinary self-inflicted wound I have seen an opposition party inflict on themselves in many, many years."
Prime Minister David Cameron said the Labour MP's actions were "completely appalling", suggesting that she was "sneering at people who work hard, are patriotic and love their country".
The 54-year-old entered Parliament as MP for Islington South and Finsbury in 2005 and served as shadow energy and health spokeswoman before taking the role of shadow attorney general in 2011.
The daughter of a former assistant secretary general of the United Nations, she was born in Surrey and was called to the bar in 1983, specialising in criminal law.
She lives in Islington with her husband, the High Court judge Sir Christopher Nugee.
She had a majority of 3,569 over the Liberal Democrat candidate at the 2010 general election.
Labour MPs said she had been right to stand down, with Chris Bryant telling the BBC "the first rule of politics is surely that you respect the voters".
And John Mann said the incident was "horrendous" for Labour.
"It insults people like me, it insults the people I know - my friends and family - Labour voters across the country because white vans, England flags, they're Labour values and actually pretty routine Labour values for most of us," he told BBC Radio 4's Today programme.
Mr Farage suggested Ms Thornberry had "looked down her nose at a white van in Strood with the cross of St George on it" and the episode reflected broader attitudes within parts of the Labour Party.
"The Labour Party hate the concept of Englishness," he told the BBC News Channel. "They have done for a very long time.
"New Labour can't even stand the concept of patriotism. They think the flag somehow is unpleasant, backward-looking and nasty. People like Emily Thornberry would rather we had that blue flag with 12 stars on it that comes to us from Brussels."