The action stems from a case of mistaken identity last year in which Hopkins confused Monroe for the New Statesman writer Laurie Penny, who had said she "didn't have a problem" with those who graffiti war memorials.
After being goaded by Hopkins in a tweet that suggested Monroe supported the desecration of memorials, the food blogger is reportedly seeking a mid five-figure sum for damages in addition to legal costs.
Rather than tagging Penny, Hopkins addressed Monroe, asking: “Scrawled on any memorials lately? Vandalised the memory of those who fought for your freedom? Grandma got any more medals?”
Monroe responded almost immediately to Hopkins' claims with a strong worded defence: "I have NEVER 'scrawled on a memorial'. Brother in the RAF. Dad was a Para in the Falklands. You're a piece of shit."
I have NEVER 'scrawled on a memorial'. Brother in the RAF. Dad was a Para in the Falklands. You're a piece of shit. https://t.co/0IGhgT5BoI
— Jack Monroe (@MsJackMonroe) May 18, 2015
The Women of World War II Memorial in Whitehall, London was defaced during an anti-Tory march in May 2015
Before they tweeted a threat to call in lawyers should Hopkins not take down the tweet, apologise and donate five thousand pounds to a migrant charity.
While Hopkins later deleted the tweet and wrote of her error in mixing up Monroe and Penny, it now appears she has not donated to charity.
"I was confused about identity," Hopkins wrote to Monroe.
Monroe has now launched legal proceedings, seeking £50,000 plus costs, Press Gazette reports. Mark Lewis, who represented victims of phone-hacking, will take on Monroe's case.
Hopkins has yet to submit a formal defence, but still has time to do so.
Jack Monroe, left, and Laurie Penny, right, were mistaken for each other by Katie Hopkins
Following the furore, the former 'Apprentice' star provoked further ire by tweeting her inability to tell the two writers apart.
At the time, Penny qualified her original comments to say she intended to provoke debate about the priorities of those lambasting anti-war protests.
“What’s disgusting is that some people are more worried about a war memorial than the destruction of the welfare state," she wrote.
Last year Monroe told The Huffington Post UK the matter has been placed with their lawyers: “Katie sent me a message on Twitter... acknowledging she had made a mistake but with no apology.
“The level of abuse she prompted her followers to send after her initial tweet went on for days and the messages I received were obscene and upsetting."
Hopkins has yet to pass comment on the legal action.