NSW independent MP Alex Greenwich on Thursday flagged he would proceed with his threat to sue the NSW One Nation leader for defamation over the "extremely defamatory" tweet posted about him in March.
A deadline for Mr Latham, who has also referred Mr Greenwich to police, to apologise for the tweet passed at 5pm on Wednesday.
Mr Greenwich said he had received a letter from Mr Latham's lawyers saying they would not accept his offer to resolve the situation outside of court and "no apology has been received".
Mr Greenwich has instructed his lawyers to begin defamation proceedings, with the filing of a claim expected in the Federal Court "in the coming days".
The tweet which has been deleted was condemned by politicians across the spectrum, including One Nation federal leader Pauline Hanson and Premier Chris Minns, who said Labor would not work with Mr Latham to pass legislation in the upper house.
Mr Greenwich has also lodged a complaint of homosexual vilification and sexual harassment against Mr Latham with Anti-Discrimination NSW which is being investigated.
The One Nation MP is also being investigated by NSW Police after Mr Greenwich complained of being harassed online.
"This is not a process I enter into lightly, and a process I wish I didn't have to engage in," Mr Greenwich said on Thursday.
"However, Mr Latham's homophobic, sexualised attack has exposed me to contempt, ridicule and extreme abuse based on my sexuality, and demands actions.
"It's 2023 and LGBTQIA+ Australians shouldn't have to put up with abuse that targets our sexuality, character, or ability to do our jobs."
Mr Latham has referred Mr Greenwich to police for possible harassment offences.
The report relates to comments made after LGBTQI protesters required police protection from hardline Christian groups outside a church in March.
Mr Latham had been invited to the church hall meeting to speak on religious freedom and parental rights.
"Alex Greenwich MP used his social media to attack all aspects of my life (as a human being) as extremely dangerous and disgusting," the former Labor leader posted to Facebook on Wednesday.
"Greenwich misused his carriage services in his attacks on me, saying that I had deliberately incited violence. In truth, I consistently condemned the violence."
NSW Police on Thursday confirmed a report had been received by Surry Hills officers and inquiries were continuing.
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