The Sunday Mirror undoubtedly set the political and media agenda with today's splash, "Tory minister quits over sex photo", but the sting does raise questions of newspaper ethics.

The newspaper appeared to have obtained the story from an unidentified freelance reporter who, on the face of it, had masterminded an entrapment by engaging in what looks to have been a fishing expedition.

Such methodology has been the subject of many rulings by the Press Complaints Commission. One, for example, stated that papers can employ such subterfuge "only when they have a public interest justification for doing so and there are no other means of gathering the required information."

Famously, in upholding a case against the Daily Telegraph in 2011, the then PCC director, Stephen Abell (now managing editor of the Sun), noted:

"The commission has consistently ruled that 'fishing expeditions' where newspapers employ subterfuge and use clandestine devices without sufficient justification are unacceptable."

The Sunday Mirror story about Brooks Newmark, the minister for civil society, did have the hallmarks of a fishing expedition.

But I understand from a senior Mirror source that executives led by the group's editor-in-chief, Lloyd Embley, spent many hours assuring themselves of the validity of the journalistic tactics the freelance had employed.

Embley evidently felt that there was an overriding public interest reason for publishing and that, despite it being a grey area, the methods were justified.

Newmark was entrapped in a classic honeytrap-style operation. He thought he was conversing with "a young Tory PR girl" called Sophie Wittams because "she" sent him "an explicit picture." In fact, it was a male reporter.

According to the Times's website here and Buzzfeed here, the freelance reporter threw the net out widely before trapping Newmark.

The Times's story is full of detail about other MPs who were contacted by "Sophie", including Robert Jenrick (Newark), Charlie Elphicke (Dover), Dan Byles (North Warwickshire, Gavin Barwell (Croydon Central), Mark Pritchard (The Wrekin) and Jesse Norman (Hereford and South Herefordshire).

They avoided falling into the trap, but it appears to have been enticingly baited with a picture of "Sophie"", and Buzzfeed has several examples of tweets sent by "her".

In its article, the Sunday Mirror reported that "the male reporter, a freelance journalist who passed the information to the Sunday Mirror, was carrying out an undercover probe into claims by sources that MPs were using social media networks to meet women."

It is very unusual for papers carrying out sensitive sting operations to rely on freelances, especially those who do not seek byline glory for their scoops.

Both the bylines were of Mirror staff: the first was that of Vincent Moss, the Sunday Mirror's long-time political editor.

The second was that of Matthew Drake, a reporter who used to work for the News of the World. He was assigned the task and played no part in initiating the sting operation.

According to the Mirror source, the fact that Newmark was a founder of Women2Win, a campaign aimed at getting more Tory women elected to parliament, and also tasked with getting more women into politics, weighed heavily in the balance of the paper's decision-making.

The source said that Newmark, as he has admitted, was quick to respond to "Sophie's" tweets. The senior staff thought this journalistic operation "fell comfortably on the right side of the line," said the source.

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