The Sunday Mirror earlier this month
The allegations were made by the journalist, who remained nameless, to the BBC's Newsnight programme.
Describing the information given by the source on BBC News, Newsnight correspondent Richard Watson said the journalist had claimed the practice was "quite widespread".
"Our source says a few people on the newsdesk and designated reporters would be doing it pretty much every day. One reporter who was very good at it was described as 'the master of the dark arts'.
"From the testimony we've got and from two other sources who I've also spoken to who corroborate this account to a certain degree, we can be pretty sure that it was happening."
But he added that the "$64,000 question" was whether management knew.
In a statement Trinity Mirror said all its journalists work within the law and the Press Complaints Commission's code of conduct.
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