Ian Paisley Jr has been strongly criticised by the National Union of Journalists (NUJ) for personal comments he made about a reporter in Belfast.

The North Antrim MP accused the News Letter’s political editor, Sam McBride, of lying as part of “an agenda to attempt to undermine and destroy the DUP”. In a 750-word Facebook post, since removed, Paisley described the journalist as “incredibly immature, intellectually weak and a simplistic fellow”.

The News Letter’s editor, Alistair Bushe, demanded in a letter to the Democratic Unionist party (DUP) MP on Tuesday that he remove the post and publish a retraction.

“I was appalled to see your egregious and defamatory personal attacks on my political editor, Sam McBride, and on the News Letter … your statements are plainly false, and although I am not a lawyer, I have no doubt they are highly defamatory of him,” wrote Bushe.

“In any event, they seem to me to be clearly designed to undermine and discredit Sam McBride’s private and professional reputations, and to harm the reputation of the News Letter (and indirectly, of me as its editor).”

In the letter, which was copied to the DUP leader, Arlene Foster, its deputy leader, Nigel Dodds, and its chief whip, Sir Jeffrey Donaldson, Bushe said that while he hoped the matter could be resolved without delay, Paisley should desist from further attacks.

“Please understand that neither Sam McBride nor I will tolerate your attacks on our respective reputations, or allow your false assertions to go unchallenged,” said the editor.

Paisley was responding to an analysis piece written by McBride on Saturday exploring the tactics of the DUP in relation to abortion reform in Northern Ireland, which will be imposed by Westminster if power-sharing is not restored by 21 October. The News Letter is a unionist paper and has a large readership in DUP heartlands where abortion reform is a divisive issue.

The DUP issued a statement on Tuesday night relating to the specific arguments in McBride’s piece, but added that “concentration should be on the analysis rather than the author”.

The NUJ spokesman Séamus Dooley described Paisley’s attack as “unwarranted and unworthy of an MP”. He said it was a “clear attempt to smear the professional reputation of a conscientious journalist”.

McBride said he was “humbled by the scale of the support” from colleagues, readers and politicians “including individuals within the DUP who have contacted me to express their private disquiet at what Mr Paisley has said”.

“It is healthy for politicians and journalists to engage in robust debate and I passionately believe in free speech. However, it in unacceptable for Mr Paisley to misrepresent what I have written, denigrate me by peddling falsehoods and for some of his party colleagues to then extend this to my family,” he said.