Sparks flew when the BBC's Radio 4 Today programme pitted two heavyweights up against each other, as host Evan Davis grilled Guardian reporter Glenn Greenwald.
Davis challenged Greenwald over his spying disclosures, which critics claim have damaged national security.
Britain's newspapers have been bitterly divided over the snooping revelations from renegade computer specialist Edward Snowden, with the Daily Mail branding the Guardian "the paper that hates Britain's enemies".
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And on Thursday morning, it did not go down well when Davis asked about reports that jihadist websites were urging caution to users, not to give out information like phone numbers online.
"What is your source for that?" Greenwald replied.
"Government officials go around whispering into the media, injecting whatever rumours they want, and then journalists who lecture other journalists about being responsible start repeating it on air.
"Tell me which specific websites."
When Davis could not name the specific website, he continued the attack: "You have no idea whether it's true or what they actually said.
"That's called fearmongering."
He went on to explain that "responsible journalism" entails not just "repeating baseless rumour".
The row came as Britain's spy chiefs prepared for a rare grilling by MPs over the NSA and GCHQ spying furore, sparked by Greenwald's reports.