The more one hears about and thinks about the Sunday Mirror sting of Tory MP Brooks Newmark, the less acceptable it becomes.

The honeytrap was certainly flawed in execution, and I'll deal with that in a moment. First, however, let's consider the flawed reasoning behind its publication.

To use such subterfuge, and I'm echoing the rulings down the years of the Press Complaints Commission here, the paper would need a strong public interest justification.

There has not been any suggestion that Newmark was breaking the law in exchanging explicit pictures with "Sophie." So the justification rests on the fact that, as a man tasked with recruiting more female Conservative MPs, he was guilty of abusing his position (plus, of course, gross hypocrisy).

Whether that passes a public interest test is a moot point. The paper's editors clearly believed it did. Others, and I'm among them, may feel it's not good enough. It's a borderline case and such decisions are, by their nature, subjective.

Let's concede the point, however, and agree that the Sunday Mirror's assessment is correct and then imagine the following scenario...

The freelance reporter who set up the sting had prima facie evidence that a junior minister who played a central role in the Tory party's bid to boost the number of female MPs was prepared to indulge in "sexting."

He realised that the only way he could prove it was to indulge in subterfuge. He baited the trap with a fetching picture of a mythical Tory PR woman, "Sophie Wittams", and Newmark fell for it.

Fair enough, just possibly. But, and this is one hell of a big but, Newmark was not the only MP to be offered the bait. At least seven other Tory MPs were also offered similar lures.

It is stretching credulity to believe that there was prima facie evidence in each of their cases too.

And what, should any of them have been as foolish as Newmark, would have been the justification if they had responded as he did? As one of the number, Mark Pritchard, points out, he isn't even married. So it would simply have been a matter of intruding into the privacy of consenting adults.

One cannot escape the conclusion that this was a fishing expedition, a giant trawl in fact, in which only one MP, a self-confessed fool, was tempted.

Incidentally, when the Daily Telegraph's journalists pretended in 2011 to be the constituents of Lib-Dem MPs, the paper did have prior knowledge of their private statements. And yet the paper was adjudged "guilty" of engaging in a fishing expedition, thereby breaching clause 10 of the editors' code of practice.

Now for the flaws. The pictures of the two women featured in the Twitter trap were used without their knowledge or permission. I suspect that could open the door for them to take legal action

Only Tory MPs were targeted. Why was that? Was there a political motive? Again, on what basis did the freelance choose his targets?

Then there is the questionable matter of relying on an unidentified freelance for such an obvious contentious "investigation". That's so unusual for such a high-profile story that I cannot remember a previous instance. It's fine to have confidential sources, but journalists should not have such a privilege.

Worse still is the use of a freelance as some kind of built-in deniability for what is produced. It was noticeable that in his defence, the Mirror group's editor-in-chief, Lloyd Embley, said it was "not a Mirror sting." But that's semantics.

The Sunday Mirror accepted the story from the freelancer, evidently tested his methodology and his bona fides and then published it. Therefore, to all intents and purposes, it was the paper's sting. It is demeaning for the Mirror to distance itself from the exercise.

I fully accept that editors must wrestle with two highly subjective concepts in such cases: the public interest test and proportionality of the subterfuge.

On balance, taking into account all of the above, I think the Sunday Mirror called it wrong.

And who is the mystery freelancer anyway? When stringers become stingers, we ought to know who they are.

NB: See also this blogpost by barrister Matthew Scott in which he makes out a case for the sting having amounted to a criminal act.

• This article was amended on Monday 6 October 2014 to correct the spelling of the fictitious Sophie Wittams.