Johnson lashed out against Moscow on Monday, calling Russia's increasingly adamant denials that it anything to do with the Skripal case "increasingly absurd."
"This is a classic Russian strategy of trying to conceal the needle of truth in a haystack of lies and obfuscation," Johnson said, as he arrived for a meeting of European Union foreign ministers in Brussels.
But some Twitter users had a different view. Calling the foreign secretary's comments "childish and boorish," users warned that there could be something more sinister behind Johnson's increasingly belligerent claims, or questioned the work of the UK's investigation into the ex-spy's poisoning.
Others simply noted they weren't convinced by Johnson's approach, given his own colorful political record.
During Monday's visit to Brussels, Johnson was expected to try to shore up European unity against Moscow over the Skripal case. "There is scarcely a country round the table here in Brussels that has not been affected in recent years by some kind of malign or disruptive Russian behavior," he said ahead of his ministerial meeting.
But Twitter users were having none of that, either, pointing out that mere months earlier, Johnson had stood as one of the champions of Britain's break with Brussels.
Monday's meeting is not expected to result in any new measures against Russia, although EU diplomatic head Federica Mogherini has said the EU stands in "full solidarity" with London over the Skripal case.