The former Labour MP for Copeland has blasted shadow attorney general Shami Chakrabarti after she claimed voters in the Cumbrian seat had been neglected by Labour.
Jamie Reed suggested Baroness Chakrabarti was "the epitome of what Labour voters just rejected" in a series of tweets directed at the Labour peer.
Baroness Chakrabarti used an interview on BBC One's The Andrew Marr Show to blame factors such as the media, disunity in the party and the poor weather for Labour's humiliating by-election defeat.
She also said Copeland had been neglected by Labour, which prompted former MP Mr Reed to fire back on social media.
He said: "Shami and co. have no idea how much the Labour vote dislikes them. Even faced with the evidence.
"She's an unstoppable vote-harvesting-election-winning machine. Or the epitome of what Labour voters just rejected. I wonder which one?"
Using the hashtag #TellShami, Mr Reed reeled off a number of projects such as new schools, hospitals and the Moorside nuclear reactors that had been achieved under Labour leadership.
Mr Reed also said the weather was not a factor in Labour's loss
Labour MP and former shadow culture secretary Michael Dugher also ridiculed Baroness Chakrabarti.
The Barnsley MP wrote on Twitter: "Pearls of wisdom from the never-having-stood-for-election, joined-ten-minutes-ago wing of the Labour Party: Labour voters "don't have cars"."
In her interview, Baroness Chakrabarti, former head of civil rights group Liberty, said: "I think Copeland is probably one of those constituencies that was neglected by my own party over some years.
"It's remote from London. It's changed its shape over many years.
"There's the nuclear industry and people who've done very well out of that industry - and then the people left behind.
"From my experience in Copeland, Labour has looked like the establishment for a very long time because they've been represented by Labour for a long time."
Baroness Chakrabarti also criticised host Andrew Marr for having Peter Mandelson as a guest the week before the by-election.
"Sometimes we haven't had the fairest or most balanced treatment in the media, including in the broadcast media," she said.
"There was a low turnout in Copeland and having been to Copeland recently, I know that it's a very rural constituency, public transport is not great.
"But it's just one factor, of course that's not the entire explanation."